Offset doesn't come from the rim, only ERD. Do not try to measure the offset in the spoke holes on the rim for calculation purposes
Offset comes from the rims relative center position to the hub flanges, and this only on the rear wheel.
A front wheel will have both sides of the spokes the same length.
The rear wheel will have shorter spokes on the drive side and longer spokes on the non-drive side (in my humble experience it is usually around 2mm to 3mm of difference between the two sides on the rear)

Added: Usually on my calculations I would round down for spoke lengths.