I agree with you here - my first attempt at disassembling a movement was with a for-repair Elgin movement I bought on eBay. Of course I didn't let the power release and once I took out the pallet fork I had a nice parts explosion lost one of the wheels forever Luckily I bought the movement with that in mind and many more can be bought on eBay for $10-$25.
if you are afraid of damaging something, leave it as is!
A beat error of 2.5 ms does no harm at all, it annoys owners of Chinese TMs only.
If you want to correct this "error", you can at least determine the direction of the needed move:
Release the mainspring fully. The now loose fork will stand a bit more near to one banking pin. In the same direction the collet must be rotated a tiny bit. Then test again with balance wheel mounted.
To get comparable values, the balance must swing with same amplitude on each test. Procedure can get time consuming like this writing!