We never have the bottom of the backstay attached when we put the mast up. The swept-back spreaders and aft placement of the shroud chainplates are usually all you'd need to keep the mast from going too far forward. Even with the backstay detached we almost always need to use a line tied between the mast ring and bow cleat to pull the mast forward enough to get the forward pin in. So, try putting the mast up without the legs of the backstay attached and with the shroud turnbuckles eased. After you get both pins of the mast step installed, the headstay on and the shrouds tensioned, then attach the backstay. If the backstay is till too short, add some link plates or toggles (westmarine .com) at the bottom of the legs, or attach shackles to the backstay chainplates and use spectra to attach the legs to the shackles. Easy-peasy.
I wouldn't trust any measurements you get from other owners since there were a lot of different manufacturers involved over the years and I am sure there were differences in the measurements.