If you're adding 50ug of protein and it's in a 20uL sample, you've loaded a concentration of 2.5ug/uL (50ug/20uL). That being said, I've never seen people refer to the concentration of a protein loaded into a SDS-PAGE - the important part is how much you load. So in this case, you should be fine reporting that you've loaded 50ug of protein per well. The only time I'd worry about volume is if you are unsure if that volume will fit in the wells without over flowing it.