UQ temp ratings.

Hi everyone. When you see a temp rating for a UQ or TW either on a website or the calsplat calc, is that assuming you are wearing minimal clothing or with some sort of base layer. Of course I understand that these temps are generalizations. I am about to start my DIY UQ and am wondering at what temp I should make it. I am shooting for somewhere around 20 degrees. I would rather add a extra ounce of down as opposed to adding clothing to sleep in to get to the same temp. So do I use the calculations for 20 degrees or a bit lower to get the 20 without clothing? This is assuming for arguments sake that 20 degrees feels the same for everyone.
Also. What percentage of overstuff is common? Is 10 percent enough? 25 percent good or too much? Would it be better to increase baffle height and lower overstuff or the other way around to get to the same temps. I am starting to cut tomorrow and am second guessing my choices.

Thank you all.