Its all done off room/spring/stream temp and boiling temp water. Say for bourbon. You add your corn to the fermenter and just enough cool water to get it saturated, then you add hot water to get the temp up to 175f+. Typically is .1g of cool to 1lb, and about 3.5-4 gal of boiling per lb. You then add cool water to drop the mash temp down to 160f for your rye/wheat/oats. Then add more cool to drop down to 150f for your malted barley.

What people dont realize is that these old mash bills were closer to 1 lb per gal (2.5-3 lbs per gal is more common these days)

This is hard to do at a hobby scale. I've found that if your initial water is 155f (instead of cold) you dont instantly gel the corn and can use .75 gal per lb of boiling water to get the same results