Once our blade is beautifully sharp, the next job is to fit it to the block (dai). We want to arrive at a fit that requires firm hammer hits to seat the blade.
Once the blade is seated we want it to stay where we put it, so a firm fit is more desirable than a loose fit.
The Blade Marks Out It's Own Place
By covering the blade itself in pencil (or sometimes a light coat of camellia oil), when we insert it into the block it will leave marks on the areas that are contacting and therefore restricting the blade.
Gently Remove The Dark Areas
We need to incrementally remove these areas in order to arrive at the desired fit. Bear in mind that some times removing a small amount can allow the blade to drop further than expected, so take this process slowly and surely.
Bear in mind that the blade can be restricted either by the sides of the channels in the block, or by the block underneath the blade. It is worth checking both.
Ideally, we want the blade to leave pencil marks evenly across the bed of the block, since this indicates and even distribution of pressure. Keep an eye out for dark areas appearing concentrated in one area, as this may indicate that pressure is being applied to one part of the blade but not others.
Minimal Cutting Edge Protruding
The cutting edge of a Japanese smoothing plane (any smoothing plane, really) should only protrude a thin hairline from the block. A millimetre or more is way too far. Sighting down the block, your eye should detect the blade as a thin dark line. Running a careful thumb over it should only tickle the pad of your finger, not stop it.
