Formula for tongue weight?

Is the following a correct way to calculate tw?
[tv rear axle (tt attached, wd disconnected) minus tv rear axle (no tt)] minus
[(front axle tv, no tt) minus (front axle tv, tt attached, wd disconnected)]

or is the following also correct?

[ (tv weight with bars engaged) minus (tv weight , no tt) ] plus [ (tt weight with bars engaged ) minus ( tt weight unhitched ) ]