But dead strings will intonate differently
There are tricks for this problem. First, leave a little more slack than you might usually, but more importantly roughly adjust the intonation before stringing. You need a measuring stick, or ruler
, for this.
Measure from the nut to the 12th, this is half of your scale length. Do not expect scale length to be exactly what is advertised in the specs, it may not be.
Next lay your ruler from the 12th fret to the saddle area. Expect the treble E string to intonate about 1mm longer than the first measurement you took (nut to 12th), so move the saddle so the string will leave it at that point.
Stagger your treble strings so they have about an extra mm per string.
Move your D string saddle somewhere between the treble E and B length.
Give your A string about 1mm more, your bass E string about 1.5mm -2mm more than that.
A good starting point for a 7 string's low B is 5mm longer than the scale length.
Exactly where the saddles end up depends on action, string gauge and construction (brand), tuning etc. but this puts you in the ballpark.