Well, Pi is 22/7. Tan gets a bit complicated. Draw yourself a right angled triangle, with one of the other angles 20 degs(or whatever AOB). Measure the length of the two shortest sides. Divide the shortest side by the other and that will give you the Tan of the angle. The bigger the triangle the more accurate will be the final answer. A triangle with sides of 42 and 114mm gives a Tan of 0.36842 as opposed to 0.36397 for a calculator.