Check whether utilizing the from to step fields can fulfill your requirement.
Example:
step 100 ZR01 (check statistical box if you don't want this to include in the calculation)
step 200 ZRX8 (check statistical box if you don't want to this to include in the calculation)
step 250 ZR10 FROM 100 TO 200
By this way, I think, you can use standard functionality and don't require any development. Test thoroughly before moving to production system.
Regards,
