First, you should measure the tappet clearances with the crankshaft on the compression stroke and rotated clockwise (viewed from the flywheel end) until the piston has descended 6 mm down the bore from its highest position (Top Dead Center). It would not matter if you moved the piston down the bore a bit less or a bit more than 6 mm: the inlet valve should be properly seated by TDC, though sometimes the tappet clearance is a bit scant until you get a bit past TDC, then nothing happens to the valves until the piston is more than halfway down, when the exhaust valve starts to open. However Briggs says 6 mm down, as a standard practice for all engines including those with decompressors, so it is best to follow that instruction.
Second, when you lap the valves you remove a small amount of metal from both valve head and valve seat, to get a smooth continuous contact surface between the two. Removing metal moves the valve downward onto the seat, which reduces the tappet clearance. Hence you should always measure the clearance after you finish the lapping job and are satisfied with the outcome.
