Currently, to recalculate a custom object with fields from another object requires either 1) manually edit each form on which the custom object resides or 2) using the Recalculate Custom Expressions, after selecting Edit, Custom Forms.
The results are: 1) neither of these options are user friendly; 2) this is also a potential risk, as data may be out of synch without the user being aware; and 3) this method is also not 'real time'.
If this design option is due to potential performance considerations, perhaps one solution could be to provide options that would allow the user to prioritize which fields to auto-recalculate?
This functionality would be really useful to have.
It would also help the PR in the client base for as you know, a lot of goodwill from other functionality is quickly wiped out with "I have to keep track of my changes? Isn't the system supposed to be able to do that?"