i just adjusted mine to suit i had a mk5 escort and went to 205/55/15 then to 205/50/15 then too 205/45/17 and all i did was pull the needle stop out let it rest it'll spring round a bit to release the tension of the spring, adjust it by eye of how much you think it needs then try again, it's trial and error but i didn't find it too much hassle, i used my sat nav as the true indication and it worked fine i never got caught in a speed camera anyway

try it on your second hand set of clocks first just incase you make a mistake.

P.s to pull the needle off use 2 spoons to prize it off without bending the pin it sits on , putting it back on you have to be careful too that you dont move it otherwise you'll be making it worse hope this helps.