....or just use the string method and some care. You'd be amazed at how well basic techniques can be when used with a bit of planning and care taken to eliminate possible errors.

When doing the string method the big problem is holding it still. Sticks of wood clamped to the frame can be used to pinch down the string while measuring so that the settings don't move. A slip of paper pinched between the string and frame and then moving the string only just far enough to let the paper slip out can be a guide for achieving the same string to tube spacing on each side. By thinking about such stuff you can get your frame aligned with string, ruler and a few bits of stuff to a degree that is more than adequite. An error of one string's width means less than nothing. It's the 1/8 inches at a time that you need to correct. And the string method with some tricks can easily best that requirement if done right.