Design for lug set

I am trying to design a bike for a given set of lugs. All of the angles in the front quadralateral are defined by the lug angles. I can set the seat and head angle. Is there a way to set the HT-DT angle and the DT-ST angle. I have found that after I set the ST length I have to use trial and error to set the other tube lengths so that the angles come out correct.

Is there a better way? I am new to this so I may be missing something.

Thanks.