The calculations I do for this purpose involve logarithms, which I do using a scientific calculator. But a simpler method, which yields reasonably good results, would be to start with the following calculator. It applies to box-type dynamic speakers, and if the calculation is being performed for planar speakers such as Martin-Logans or Maggies that are listened to at typical distances (such as yours) add around 5 db or so to the results it provides. The reason being that as listening distance increases SPLs fall off more rapidly for box-type dynamic speakers than in the case of planar speakers. (And in fact the volume produced by a planar speaker may be greater at say 2 meters than at 1 meter, since the upper and lower parts of a planar speaker will generally be more on axis at 2 meters than at 1 meter).
https://myhometheater.homestead.com/splcalculator.html
Best regards,
-- Al
https://myhometheater.homestead.com/splcalculator.html
Best regards,
-- Al