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Paul, Tom is correct, the isosyncronism of the balance is directly related to how accurately it maintains the time, as well as many other factors.
Just to get it straight, you’ve poised the balance with the roller table in place? What I usually do is after I poise, not pose, that’s later I then set the completed balance in the movement. Some set the roller jewel in the neutral position, I set it so that it is leaving the pallet exit stone, then as Tom suggests, I loosely position the cock, love that word, then adjust the hairspring, via the collet, to its mating position with the balance cock. All things being equal, it should be in good beat, which also effects the timing, to a certain extent.
I’ve seen very slow balance iso on a watch and it has gained time, also the contrary, so yes, it does have an effect on time keeping. Also maybe your watch has a replacement HS? A weak mainspring and have you demagnetized it? Did you polish and closely inspect the pivots etc, test for end shake.. I’m just stating the obvious because sometimes the small things get forgotten.
You should not have to adjust the hairspring stud as this should have been set to match the wheel, unless you suspect it has been replaced badly.. Also make sure the two little brass pins that the HS runs through are not too tight or loose, and that they are clean between the two pins.
What are your watch specs Paul?