I set tun loss at zero but my tun sucks out almost everythign when testing with water. It leaves about 1/4 cup.
20% boil off is what I used to have when I used a pot. If you have a faily wide pot (if it's about as tall as it is wide) then 20% is probably right. After a couple of years of using the pot (it was a ten gallon megapot) I switched to a keggle. The keggle only boils off about 15-16%.
Theres nothing wrong with the 20% boiloff. You just have to plan and increase your pre-boil volume accordingly.
Up next: Kolsch, Alpha Squirrel, Tripel rebrew, Witbier
Currently Fermenting: Charlie Sheen (barleywine)
Kegged: Tiger Blood IPA, Irish Red, Strict Observance Tripel
Bottled: Dry Stout, Pliny the ..... Squirrel?, Cream Ale