It's going to depend on whether you are using a pot still or a column still. A good column still will output ~95% ethanol, so the collected volume will be:
Collected volume = Starting volume * Wash ABV / 0.95
Thus if you start with 3 gal of the following ABVs, the residual volumes will be:
10% ABV: Residual volume = 3 * (1 - 0.10 / 0.95) = 2.68 gal
15% ABV: Residual volume = 3 * (1 - 0.15 / 0.95) = 2.53 gal
20% ABV: Residual volume = 3 * (1 - 0.20 / 0.95) = 2.34 gal
If using a pot still, your output will start fairly high (~80% ABV) and drop off as you distill, so calculating the residual volume gets much more difficult. If we assume that the average ABV of the collected product is 50%, we can use the above formula, with 0.95 replaced by 0.50. The lowest residual volume for 3 gal of 20% ABV wash would then be:
Residual volume = 3 * (1 - 0.20 / 0.50) = 1.80 gal
Looking at some data I got from somewhere, the average ABV collected from 3 runs was 60%, so 50% seems like a good worst case value to use.
In any case, the residual volume will be your starting wash volume minus the collected product volume. So, figure out what minimum volume you need to cover your element, and don't collect more than starting volume minus minimum volume, and might be good to put a safety factor on that number.
Brew on