How would you ask your employer for a raise?

I am sorry to say but there is a huge difference between take home pay and gross pay. You can not compare the two there are too many variables. Also expecting to make the same as a branch manager who had been there for a while is not wise. Most companies have minimum and maximum salaries for each slot. Since you are a new branch manager they pay you at the low end of that scale. What I would do instead is remind your boss that you need your three month review for being in your new position and with this current economical environment most places are doing wage cuts not raises.