The important thing is that the newly exposed parts should be the same width as the remaining part of the original sides.
The traditional way of marking it up is to use a spar gauge. This will 'magically' draw the lines the right distance from the edges. This is really only necessary for a tapered spar. It just implements the 1/√2 rule that comes from cos 45°, or Pythagoras if you prefer.
Then it was a matter of planing the corners off until I was happy enough. I'm not making wheels here so roundish was good enough.
Now I have four thick discs.
Three steps left to go.
- round the edges - this worked well with a 'rounding-over' router bit last year.
- cut the groove for the wire rope to fit in - a 'core-box' bit does this nicely.
- cut the holes for the rope.
The holes for the rope are strictly 'blind sheaves'. This means they are doing the job of the sheaves (pulley wheels) of a block, so they should be as round as possible too. I'll have a go at using the drill at three 45° angles to make that octagonal (5 sides of an octagon) too. Then I'll try using abrasive cord to take the corners off.