Well, I've never done exactly what your doing but It looks to me like the same problem I have cutting biscuit slots.
I put a piece of masking tape on the table and mark the lead and trail edges of the bit on the tape.
I then mark the starting and stopping points on the edge of the board.
When a starting mark lines up with the lead edge mark, I plunge into the bit and move the work until a stopping point lines up with the trail edge mark on the tape.
Huh, hope I was clear on that, seems a lot harder to explain than to do!
Cheers