I built the frame script from the cranks out; there are several systems which are used to communicate frame dimensions so this is just one approach. As of V6 you can expect a considerable amount of trial and error but the key component locations are modeled.

Imputing a sample table like:

You get a result of:

Forget the extra tubes, think of them as boundaries.

There is a pretty big error right now, this system relies on fork length to drive the top tube length. But that's the status of the builder so far, working on V7 now.