One measures inseam to get a number to put into a calculation for something. Therefore use the method specified for the calculation. If no method is specified for the calculation, do some more research. There's a method prescribed for every fit calculation, or if no method is prescribed, don't use that calculation - waste of time.

I think the most used calculation is the

Competitive Cyclist Bike Fit Calculator, though it often raises more questions than it answers, i.e. there's no one correct formula for all bikes and riders. It does give very good directions for how to take the measurements.