Originally Posted by ViperZ
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.
When I get home I was going to sit it on top of some old wheels.
It's custom geometry so I don't know if it follows all the rules of traditional frame building. A lot of Sevens frames have a couple degrees of slope in the top tube. So I would think this is considered sloping.
I measured the seat tube and top tube myself.
I'll have some answers in a couple hours.