Question 250892
Look up the Law of Sines and the Law of Cosines. I am not sure if you can do what you are asking the way that you are explaining it.  However, if you would like to discuss it and talk through it, then I will be happy to help.  It will be hard to measure a diagonal without knowing the base and height, but you can project a diagonal at a specific angle and If you need it to be a right angle, then it MUST meet the criteria for the Pythagorean Theorem.  For example, if a^2 + b^2 =c^2, then it is guaranteed to be a right triangle.

If you want to work some problems on an online whiteboard, then send me an email and I will be happy to help you.  We can work out this problem and others until you feel comfortable with doing these on your own. We can also talk over the phone or through skype. 

All of this is provided on a volunteer basis and is first come first serve (although I haven't had a huge acceptance of this offer so there is a good chance I will be able to help you right away).

justin.sheppard.tech@hotmail.com 
Respectfully,
Justin