You can calculate the amount of troops you would need to fill up your farm minus that. I don't think that is really a feature worth adding IMHO. I usually have a set amount of troops for snob trains, and then have a different set amount of troops for a full D & O village.
True, I see where it can be useful, but before constructing, calculating buildings and troop counts should be done first before your are complete building your village. The dev might add that feature, does sound like a good feature if you are adjusting your village specs.