Stack & Reach: Calculator

Came across the following calculator for stack and reach while searching. I’ve tried to plug in a few bikes form various brands (i.e. Cannondale, Specialized, etc) who provide measurements. Seems to get close but not exact???

http://cheapcarbonframes.com/wp-content/uploads/2010/08/sizing.html

is the stack more accurate than the reach?

effective top tube length would be hard to consistently measure across some bikes

No, for example…tried to plug-in the information for a 58cm Specialized Tarmac SL4 (Stack: 59.1 and Reach: 40.2) and got:

Stack: 60.2

Reach: 39.7

Assuming a standard dimension of 368mm for the “front axle to fork crown”? Most brands don’t seem to provide this information, so any idea how you might be able to determine an accurate front axle to fork crown measurement?