Reconcile exposures in LR4

How can I adjust a couple of bracketed exposures in Lightroom so that the exposures match?

Let's say I take a shot at ISO160 and 1/100th. Another shot at ISO200 and 1/250th. Different and unknown apertures (manual focus lens). Is there a picker tool in Lightroom to tell me that "This spot here has a Value of X" so that I can compare the same spot on the two shots? I want to adjust the exposure until they match.

I'm shooting in RAW and I don't use Photoshop, only GIMP on occasion. I've found how to use the picker tool in GIMP but I can't edit RAW files in GIMP. Supposedly UFRaw will allow this but I'd prefer to do this in Lightroom.