Sorry, for the long winded response, but I just want to be as clear as I can be...

Finding the tangent of the angle, you just enter the angle value into a calculator and hit the 'tan' button. What tangent means is this... it's just a ratio of the height of the cg divided by the distance from cg to rear contact patch.

So in the attached diagram, say your cg is 18" off the ground and 90" in front of the rear contact patch. Tangent of an angle = opposite side / adjacent side

So tan a = 18/90 = 0.2 Now hit "tan-1" button on a calculator and this gives you an angle of 11.3*. This is without a wheelie, this is just the natural angle of that chassis at rest.

Now, as your front end rises in a wheelie, the horizontal distance from contact patch to cg will get smaller and smaller as it comes closer to passing over the rear wheel. Obviously, the height of use cg will increase as well as it rises.

So, here's the answer to your question (finally!). say you measured an angle of 30* during a wheelie. You would enter that into a calculator and hit "tan". This would spit out a number (0.557). This is the new ratio of height of cg to horizontal distance between cg and rear contact patch. Then enter that into the equation and you're done.