According to my workshop manual 18RG bucket clearance should be 2mm approx, both inlet and exhaust. Intake valve clearance 0.24-0.34mm. Exhaust valve clearance should be 0.29-0.39mm.
Calculate thickness of new pad so valve clearance comes within specs
T1 **Thickness of pad used
A **Valve clearance measured
Intake side
New pad thickness
=T1 + (A - 0.29mm) 0.29mm being desired clearance
Exhaust Side
=T1 + (A - 0.34mm) 0.34mm being desired clearance