Remember regulation and adjustment was practiced long before vibrographs were invented. You can just remove the balance and note the position of the impulse jewel with the hairspring at rest, then center it in relation to the pallet fork. You can get it 90 to 95 percent close via this method. Maybe even 100 percent, but no guarantee on that without the vibrograph. Adjusting the timing can be down on a trial-and-error basis over several days as you have demonstrated; the vibrograph just eliminates the trial-and-error process by providing instant results.