Ah, but you say the fields exist in separate tables. You can use either the field calculator or update cursor in a python script, but the thing here is, you can loop over a list of fcs you want to update the field for and run the same field calc.
You did not say if you needed this in modelbuilder, but I suppose the same thing could be accomplished with an iterator. I have found usually this is just as quick and as easy to set up via Python and there are plenty of web help samples to follow, such as at the provided link last post for the update cursor. Hope that helps.