I believe that whenever you get a bonus they are required to withhold at the highest marginal rate (nearly or over 40% when you include state withholding). You get the money back when you file your taxes (if you end up being in a lower tax bracket).
If you really want to know there are websites showing billable hours and salarys, like greedyassociates.com. It seems dumb to me though as you have several years to go, you probaby will not get a big law job (all 1ls think they will, most do not). If I were you I would focus on my current classes not how much you are not going to make.