Macro - Rear screen too dark

[bes]

A noob question. I am using a canon 1100d with a reverse mounted lens for macro. The camera is in manual and I focus by moving forward and back. While snapping away without a flash there was no problem using the rear screen for focusing because I had to set the exposure more or less correctly. However, now that I have a manual flash to play with I can up the shutter speed but I can't focus because the screen is too dark. Is there a setting I can use to brighten up what I can see out the rear LCD regardless of shutter speed?