I've seen two approaches used by techs to calculate the heat load of my 50-60 year old home. Two of five mentioned Manual J or an equivalent software. The others
said that since anyway there's no way to estimate accurately the R value of the insulation, they use the square footage with some formulas. So far I got 3 tons and 3.5 tons estimates.
Interestingly enough, the 3.5 ton value was quoted by two techs using the two different methods.
How to decide which is the right size, and suppose we go with the 3.5 t and it's indeed oversized, how bad would it