Valve stem lengths

Help
Seats are cut, Gp 1 valves are fitted, stems are to long
Other than trail error does anyone have a trick or formulae to work out
much to shorten them first time ??
Thanks
Chris
Seats are cut, Gp 1 valves are fitted, stems are to long
Other than trail error does anyone have a trick or formulae to work out
much to shorten them first time ??
Thanks
Chris