Leeroy_25

So my 1994 FD3 rev Counter (Tach) when all crazy town on me but I have actually managed to get it fixed by the look of it. At least partly. After replacing the Capacitors and re-soldering everything. Mainly the chip legs I think. The Tach now reads nice and steady. only issue is it appears to read about 500rpm high if the readout from my microtech ECU is good. Which I assume it is? So maybe the variable resistor or what ever it is on the back got knocked during soldering and refitting and so on.
Does anyone know if there is an easy way to calibrate this out of the car as there is no way to get at it when it is fitted other than that it will be guess work adjustment and I don't know which way or how much to move the screw top to start with? Any help on this would be great please.
Thank you
Lee