I don't usually pay much attention to the math of selecting wheel diameter. Sometimes I just use what I got, which by the conventional wisdom is usually way too small. Like @bluesman7 said, the grind does not necessarily precisely reflect the wheel diameter. In a perfect world I would have two professional grade 72" x whatever grinders in twin configuration with lots and lots of contact wheels. The reality is a bit different and I have clamped my share of belt sanders upside down in the bench vise. A twin grinder is on my list of projects to do. Unfortunately it is a long list. When you are retired, you never get a day off.