Learning to Cut to a Line

My friend Brian says that great woodworking requires cutting to a line accurately. I don't think it matters whether the accuracy is by hand or by machine, but I know that he was talking about hand work.

He suggested an exercise for learning accuracy with a handsaw, one I found similar to playing scales and arpeggios on the piano: deceptively simple, repetitive, but challenging to perfect. I've started doing this exercise whenever I have a few spare moments in the shop:

1. Mark a shoulder line 1" from one end of a wood scrap
   This length amplifies error enough that you can see even a small divergence from the line).
2. Using a square, mark lines from the shoulder line to the end of the board
   Make as many as you can fit across the scrap—Brian suggests putting them 1/8" apart, but I've been marking them further apart than that).
3. Wrap the lines onto the end
   This exercise leaves me wishing for a saddle square: I've found that error creeps into the intersection with almost every set of lines—it isn't obvious until I cut to the line, but then I can see that the two lines tend to be as much as 1/32" off.
4. Cut to the shoulder following the lines
   Check the resulting kerfs for square both across the top of the board and running down to the shoulder line. Initially I was cutting on the line and realized that this prevented me from judging how accurately I was cutting to the line. Then I tried cutting next to the line, which gave me enough visual feedback, but would require chisel work to complete a joint. So finally I conclude that I should strive to cut the line in half. This leaves enough pencil to judge accuracy, and when I'm accurate removes all of the waste.
5. Cut off the kerfed part of your and start again with #1
   Rinse and repeat. Brian's teacher was required to do this 200 times before being allowed to cut dovetails. I've only done it twice, and it seems like it might just take 200 times before I am accurate with this.

This exercise develops at least three skills required for cutting good dovetails:

1. Cutting square to the board end
   Tails need square cuts. And if you cut tails first, this allows you to cut matching tails at once.
   
2. Cutting to the shoulder line on both sides
   Cuts past the shoulder will show.
3. Following a line
   By following the layout lines exactly, you end up with tight joints that didn't require a lot of adjustment with a chisel.

I hope to find that this exercise has improved my dovetails next time I use them. First I'm learning the hard way how to flatten a board with a hand plane: lesson 764 in woodworking - S4S does not mean flat.