What is good water-pan placement for offset smoker

I have a couple offsets, I put my water pan on the grill grate closest to the firebox. This does 2 things, it's easy to replace the water after it all evaporates and it adds a block to keep hot firebox direct heat off the meat.

According to Meathead Goldwyn, water pans can go over the heat source or under the meat, depending on the style of cooker you are using, and the desired effect. They can aid in maintaining temperature or adding moisture to the environment. I would say, in your case, to avoid placing it under the meat, as there will be quite a bit of air circulation in your offset smoker.

I don’t have a smoker like yours, but I have one of those drum style charcoal smoker/grills with the chimney. I smoke my Boston butt on indirect heat by putting it on a “V” shaped rack. I put the rack in an aluminum pan and put the water in the bottom of the pan because there isn’t a lower rack on which to put a water pan. The meat/rack/pan is on the grill grate on the left side and the fire with the wood chips is in the bottom of the right side.

I usually rotate the meat after some time so that the heat is evenly distributed. It usually turns out great! Be sure you have a meat thermometer so that you get the meat to just the desired doneness.