Sorry I wasn't any help, but here are a few more thoughts.
Your question was asking what the lowest signal two or more AP's on the same channel should be receiving, if at all. The thing is in a Wi-Fi network design this is a factor that is very difficult to take into account.
Once you determine how many users each AP can accommodate then AP placement is determined by coverage requirements. With an enterprise environment your AP overlap will vary due to the different attenuation that you will experience.
You can have too much power overlap so adjusting the power is the answer. But again, the power can never be set perfectly since there are only three available channels. You will almost invariably experience more overlap with some AP's vs others.
(Quote from Joel):You could also prohibit clients from using low data rates (1 and 2mbps for .11b, 6-12mbps for .11g and .11a). This will decrease the range at which clients "see" other radios but you'll need to make sure that doesn't cause problems with legacy equipment that may not fully support that operating mode. (End Quote)
Keep in mind that when you disable the lower data rates you are only adjusting what clients "see" as Joel pointed out. It does not however limit how far the RF travels. So, you can limit the effective range of an AP by disabling the lower data rates but it does not limit the actual distance that RF travels like an adjustment to power output.
Good luck in your endeavors.