How do I properly substitute flour and water for sourdough starter?

Starters are typically maintained at 100% hydration. That means equal parts water and flour. So, in your case, mix equal parts water and flour. Measure 1 cup of that, and add it to your mix. Of course, this will mean that all of your starter is gone. Alternately, feed your starter (equal parts water and flour) with more than you need, and let it sit on counter over night. Then you you can save the left over to use again next time.

Most starter recipes are using equal parts flour and water to cultivate the yeasts and lactobacillae. So yes, if you have to "top up", use a mix of half water, half flour.

I hope you are not going to use all of your starter for your bread, but are using some of it to keep cultivating it? A tablespoon is actually enough for a cup of water/flour slurry... Note that most starters need a few weeks (or months) to reach their full potential and good balance of yeast and bacteria.

You've got another problem - you have less than 2 cups of starter and you're about to use it all?

The answer to your actual question is as follows:

You can't really do it this way. This is a good example of why weight is a more useful measurement than volume in baking. A cup of inactive starter will weigh more than a cup of bubbly, active, rising starter, because the active starter has lots of air in it. Adding flour and water will increase the weight, but not the volume, and when you're using sourdough starter to bake, it should always be bubbly and active, usually about 3 hours after its last feeding.

Solution:

The next time you feed your starter, take the portion you remove from the main batch, weigh that removed portion and put it in a bowl. Feed the main batch as usual, then feed the removed portion too (as a general rule, the feeding process involves equal amounts by weight - e.g., 4 ounces starter gets 4 ounces flour and 4 ounces water, but you can increase the amount of flour and water a bit without causing problems). Loosely cover the bowl with plastic wrap or foil or a clean cloth, then let it sit at room temperature until it is very active, bubbly, and rising.

A cup of active, ready-to-use starter should weigh roughly 7 ounces (~200 grams). A cup of flour weighs about 4 1/2 ounces (120 grams). A cup of water weighs about 8 ounces (~236 grams). You want to end up with about 14 ounces (~400 grams) of starter.

For your specific needs, when you put the removed portion of the starter in the bowl, subtract iits weight from your desired total (14 ounces/ 400 grams). The resulting number is what you have to add - half of the weight in flour, half in water.

For example: If you put 3 ounces of starter in the bowl, you need to add 5.5 ounces (160 grams) of flour and 5.5 ounces (160 grams) of water to it.

Wait about 3 hours or until the starter in the bowl is bubbling happily, and has visibly risen a bit. Now your starter is ready to go, and you can either measure it out with a measuring cup to make sure it is 2 cups, or weigh it to make sure it weighs 14 ounces (400 grams).

There's a better way to do all of this next time you want to make something with your starter: the last time you feed it before you start baking, skip the part where you remove some of the starter. Just feed it with the amount of flour and water you normally would, wait a few hours for it to bubble and rise, then take what you need for your recipe¹. You'll have enough for your recipe, and you'll still have starter left for future use.

Notes:

¹ There's a reason you're supposed to remove most of the starter before feeding - if you just keep pouring flour and water into the starter without getting rid of some first, the amount of yeast and bacteria increases until the amount of flour and water isn't enough to feed them all. It makes the yeast and bacteria less active - and less able to leaven bread - and it throws the starter's ph levels out of whack. You can't feed without removing some starter every single time you feed it, but doing it every now and then when you are about to bake with it is fine.

Do not use all of your starter.

You should not have to "waste [your] starter" when making the bread, and generally speaking, starters require very little maintenance to keep alive. The main thing you have to do is use it occasionally and keep the consistency right (kind of like pancake batter, maybe a little more runny).

If you do not have enough starter, just feed it some more.

It is generally bad practice to use up all of your starter, because how will you make your next loaf? Just add some flour and water to your starter the day before and presto, you have more starter. This isn't even usually necessary though, as you can add a greatly varying amount of starter to your bread as it will leaven your dough over time. If you used less starter than your recipe called for, just let your dough sit a little longer.

Sourdough is an art, not a science.

If this is your first time using sourdough, which seems to be the case, I highly recommend finding someone versed in its use, and learn some tips from them. My bread "recipe" is more like a suggestion, but I change it every time based on how my sourdough seems to be doing. Humidity, activity, and flour type are all factors that can influence how it behaves. That being said, sourdough is easier than you might think - it's hard to mess up, but getting that perfect loaf just requires experience and an intimate knowledge of your starter. Give it a go and don't worry too much. If it doesn't turn out exactly like you wanted, make some adjustments for next time. Just don't use all of your starter so there will be a next time.