You may wish to reassess whether you even need to build a new calculator. plenty of others have already built free and easy to use calculators built against GHG

You may wish to reassess whether you even need to build a new calculator. plenty of others have already built free and easy to use calculators built against GHG Protocol for non-complicated GHG scenarios like the group you described, like here https://smeclimatehub.org/start-measuring/ https://smeclimatehub.org/start-measuring/