Rounded Wt. + [Actual Wt. x (B.A.-Datum)]/(Moment Constant) + Datum Constant.
.. if this is one side of an equation, the units are inconsistent - something's wrong.
There are no curved portions of the envelope since all the variations are straight-line.
The only WT/IU envelopes with straight lines are the simplest seen with very small aircraft. Otherwise, generally, all have curved portions. Then again, you may be using tabular data to input only the final WT/CG in which case the envelope will present as straight lines for most aircraft
Need to see what you are playing with to offer further comment.