Gear inch is the diameter of a hypothetical front wheel from high-wheeler, that will travel as far in one pedal revolution as your bike will. Gear used to be adjusted by buying a bike with a bigger front wheel.

The distance it will travel in one rev, the "development," is the gear inch number multiplied by the constant PI, equal to around 3.14, or 22/7.

The formula for gear inch is GI=wheel_diameter * chainring_teeth / cog_teeth, so yes, you had it real close. I don't think it needs to be extremely precise, so if you measure the wheel diameter with the tires inflated to 1/8 inch with a yardstick, I think you have good enough data for studying the spread and stepping of a gear system.

There are some standard ISO values for wheel diameter for all the standard tire types, but I'm not sure those are true for all commercial tires. I'd recommend just getting out a yardstick and measuring. Besides, the ISO documents cost significant $$$.

I know Sheldon recommended using "gain ratios" but I'm not sure that number helps. It doesn't help me, at least. It's just a different way of looking at measuring gear.