Sloping top tube?

TBH it doesn't sound like you should be designing your own frame? Perhaps talk to whoever is building it about what you want, and let them figure out the geo?

I won't, they asked me to decide the main lenghts/angles, and because the builder is outside the UK, this is my only way of doing iT.... Just decided to give it a go, they will never fully compromise with what i've done here.. Thanks for the advice ; )
Still i would like to understand what they'll do, if I can.