Do the regulators have a formula or acceptable percentage mix of branch locations by census tract levels?
EX: X% in low, X% in moderate, etc?

At a past Regulators forum, they mentioned that they don't expect Banks to keep branches open just to have a presence and that expect branches to be profitable.

There is no "rule". Regulators will look at this in a performance context type of manner, like lending in every tract. You are not expected to have a branch everywhere, but if it appears that the bank is avoiding certain tracts for no supportable reason, be prepared to be asked. All areas should be considered and decisions as to why branches are and are not placed in areas should be well reasoned and documented.

You certainly don't want to be seen to be "circling" a low moderate area and never entering it. Also look at proximity of other branches to low/mod areas...are they placed that they can easily serve the low/mod area? If that is the case, that fact needs to be reflected in your documentation and performance context.