Why 'start' anywhere but tested best?
If he pulls seating out of his butt, he certainly risks disappointing final results, and any revisit of seating will collapse his powder efforts to that point. Around here, they call it tail chasing..
He needs to take his brass to stable dimensions anyway. So he might as well perform Berger's seating testing while doing so. With this he can proceed with at least rough seating, which will be far better than worse possible seating(as pulled from his butt).
Bullet seating is the single largest adjustment in load development. With seating, in the wrong place, you can quadruple group size. Powder can no more than double group size(or cut bad seating choice in half).
And with any calibration, you adjust coarse before moving to fine.