Use some basic geometry to measure the effective length.

You can use: http://www.carbidedepot.com/formulas-trigright.asp

Angle A = (90 - HT angle) + stem rise [for example, for a standard 73-degree head tube, a 10-degree rise stem would be a 27, flipped over at a negative 10 degree rise it would be a 7]
Side C = stem extension

Calculate...Side b will be the effective length coming straight out from the steerer tube parallel to the ground.

I use this site to figure which stem I need to change the effective length and height of the handlebars with a new stem. If you run the geometries of two stems, you can see the difference in both reach (side b) and height (side a).