How much calcium propionate should I use as a bread preservative?

Most sources recommend 0.1% or so, but not more than than 0.5%. Try to use only enough to achieve the preservative results you desire.

In other words, for 1000g of flour, start with 1g of calcium propionate. If that works good for you, then stop there. If you still think it's spoiling too soon, then next time try 2g of calcium propionate for 1000g of flour.

Adding more calcium propionate may or may mot make it last any longer, so don't use more than needed — and never more than 5g for 1000 of flour.

ElmerCat has it mostly right -- 0.1% to 0.5% of the flour weight. Why such a wide range? Because the need depends on factors other than the recipe. No matter the recipe or environment, when fully baked bread comes out of the oven, it is mold-free. Mold simply won't survive the baking process. It's what comes later that introduces mold to the bread -- either through handling or exposure to mold in the environment (which could be air, smallwares, knives, cutting boards, containers (bags or breadboxes)... It's all around us. So, the unseen and unknown amount of mold exposure determines how much calcium propionate you'll need. For most situations, it'll likely be from 0.2% to 0.3%.

The calcium propionate powder that I recently bought weighs, on my scale and using my measuring spoon, at a shade over 0.625 grams for a quarter (1/4) of a teaspoon. (Your results may vary. I recommend checking your weights first, if you have a milligram-capable scale. If not, 1/4 teaspoon of calcium propionate isn't likely to give much mass, in any event, and such small amounts are unlikely to have a detrimental effect either on your health or the quality of the bread.) Using that much per pound of flour works out to 0.625g/454g, or 0.138% of the flour weight. That is where I'll start. I have some dough rising right now. (For what it's worth, I whisked the powder into the flour before water was added. I would imagine that it could be dissolved in water, instead, for sure-fire even distribution, but that's extra work and the amount of mixing and kneading of the dough should suffice, in my mind.) That batch should make eleven or twelve English muffins. Usually, I'll freeze about half the batch to prevent spoilage; this time, I'll keep them all in the pantry and see how well they last. (Tip: I've taken to adding lecithin to my bread doughs to keep the final product moist longer. It works well -- but should also provide a more favorable environment for mold growth. I think this will make for a good test of the effectiveness of the amount of calcium propionate that I use.)

I hope this helps.