It really is best to use thin string or cotton on the two E strings to centre the bridge, rather than try and work out a centreline with a laser etc. You definitely know then that the strings run parallel with the sides of the neck. That’s what you want to achieve, so you might as well use the method that tells you that.

Make sure the saddles are wound almost as far forwards as they’ll go when measuring the scale length, none of this ‘halfway’ business that’s in the guide.