In my opinion, The amount of light gaps, of a certain sights pair, differ according to the handgun (sights' radius) and the person (arms' length). It is also pretty much personal preference.
However it's better to open the rear notch. The front blade should stay thick in order to enable easy pick up in a hurry. It's also the very reason to prefer large light gaps rather then too narrow ones.
I can recall reading somewhere Jeff Cooper's notion about front sight to be 1/10 to 1/8 inch thick. IMHO and experience it should be at least 1/8" for fast shooting.
[Edited by ctrshot on 03-16-2001 at 05:01 AM]