First, use some prussian blue and observe where the old seat contacts the valve face. If the old seat is on the valve face with no overlap, then proceed with the simplified procedure below: Use a 15 degree stone and cut the old seat in half. Grind the new seat. You want intakes to be the width of a dime and the exhaust the width of a penny (nickel in the case of auto engines). Finish by using grinding fine grit grinding past to lap the valves. Check your work by pouring solvent into the inverted combustion chamber with the spark plug installed. There should be no leakage through the ports. You will find the new seat meets the valve face in the same area as the old seat. This procedures saves countless trial and error grindings to get the seat in place at the correct width.