I would set my saddles in the middle (ish) of their adjustment range, and then use that middle position to measure the 25.5” distance from the nut.  
For other bridges, placement measurement is more important because there’s less scope for moving the saddles in order to get the intonation set correctly.

Newbie planning on a build, but I'm not a guitar player. It's for my son. How important or should I say is there a finite measurement for the placement of the bridge in relation to the bone nut....? 

I have seen and used the fret calulator of the Stewmac site, and I have from that worked out that from the bone nut (inside face) to 12th Fret is 323.85mm, so for  25/5 scale doubled is 647.7mm.  So am I to conclude that the bridge should be exactly in line where the strings terminate at 647.7mm or there-a-bouts. Only within the image text  (Attached) it says that the string length won't match the scale as the saddle position varies..  not that I fully understand that really if I'm honest.

So does that mean that effectively I could put the bridge further away/or closer than the 647.7mm measurement. I'm a little confused.  🤨

 Excuse the crued drawing but think it explains what I am asking.. 

Appreciate an expert help, cos I'm struggling with this one.

TIA

Couttsy