It's basic math, but that is only necessary if you are designing from scratch and have a force arrangement in mind.

You do still have the plans for the airplane? It's so easy. Just draw a couple of lines on the plans and measure the distance. Generally plans are drawn with the zero line aligned with the edge of the plans, like a boarder drawn around the plans. If it's there fine, if not, then you may want to draw a line with the horizontal stab if that is zero. Then measure the distance from the leading edge of the wing and also the trailing edge.

Now block up the model until the tail is the same distance from a table, front to back. Then just measure the wing distance, and the difference should be the same as on the plans.

But really, with your computer you can look up the trigonometry needed, and use the scientific calculator on your computer to do the calculation.