We came across this tip by T.D. Culver, Cleveland Heights, OH in Fine Homebuilding
Lining up the blade of a miter saw with the cut mark on a piece of molding can be difficult to do perfectly. Rather than sneaking up on the mark, try this trick.
Put a piece of masking tape on the saw’s table right over the slot where the blade extends below the table. Blue tape works well here. Now slowly lower the spinning blade through the tape. The resulting blade-wide slit in the tape shows the exact placement of the blade at whatever angle you set the saw. When the tape gets ragged, apply a new piece.