Insomnia rules and grid rules

I suffer badly from insomnia and have done for several years now. I don't take pills except for a 10 day course two or three times a year to help me establish a sleeping pattern. Some times this works for a month or two gradually getting worse, sometimes it starts immediately after the course. The last couple of weeks have been exceptionally bad particularly when the hot flushes are factored in .

Enough of my moan now onto ruling out a diamond grid.

I have a square with about 66" sides with a circle with a diameter of 60" right in the middle. I marked off 1½" along one side and then marked lines going in two directions at 60º This has worked quilt nicely for the first half. But obviously I shall be working off just three a inch wide grid at the sides of the circle to do the 2nd side.
My longest ruler is 39". It works out quilt nicely that the lines are 1¼" apart. As you can imagine I only need to be 1º or a 1/100th of an inch out for this to multiply out of control where the grid meets on the opposite side.
Keep you fingers crossed.......