How do you convert ssmh (inv 228.06 / rim 243.80) and convert into standard footage. I was told just subtract the invert from the rim but I'm not sure.

Sounds like top of a sanitary sewer manhole frame (equals top of MH cover) to elevation of lowest pipe and I assume you are looking for depth of manhole.
243.80 minus 228.06 is 15.74 feet or 15 feet and .74 x12 = 8.88 inches which can also be expressed as 15 feet 8-7/8 inches, if that is what you mean by standard footage.