Originally Posted by bbp
I'd have to go home and double check if the top tube slopes. If anything it might slope a tiny bit. The head tube length is 10 cm. I really don't know why the original owner had it made like this. How do you measure the virtual seat tube length?
The first thing is to ascertain if it is a sloping or traditional frame. The virtual measurement would take a bit of extrapolation with a tape measure and a level.
If it was me I would put wheels on it, so I could get the bike level. Once there I would run a bubble level from the head tube/top tube junction back towards the seat. Level the bubble level and measure from the bottom bracket to where the bubble level intersects the seat post/ seat tube area. You may need an extra set of hands.
Did you actually measure the seat tube currently to be 56 cm Center to Top? Again unless it's a sloping frame, that head tube looks too small to be a 56 cm frame. However I could be mistaken.