Measurement comfort? need to bring in a rectangle the length is 39'7 and...

need to bring in a rectangle

the length is 39'7 and the width is 15'10 beside diagonal measurements at 42'8 (in order to product it a perfect square)

however when I stack out the measurements the diagonal is never 42'8 other like 39'8 or 43'2 but never 42'8, what am I doing wrong?
Need to alter pane to landscape one means...
Since the diagonals are bad, you need to bring the corners together on the long diagonal and apart on the short diagonal until the diagonals game your 42'8" distance. I suspect what has happen is the string is stretching on one side or the other. It is only a 1/2 or so within any direction to throw the diagonal off that far. In effect your corners are not faultlessly square, you have made a rhombus not a rectangle. You could win a longer tape weigh up.
How do you obtain out ball-point pen ink...

Can I retrofit regular toilet to be a...
If you're using a typical butt joint later you have to consider the bulk of the wood being used and how it it after added to the length or width (depending on which side the wood be placed. For instance, if you were using 2x4s and added the boards to the existing 39'7" you'd hold to add an supplementary 1.75" to both sides to accommodate for the actual thickness of the 2x4.

As long as parallel sides are of the exact dimensions and the cross measurements are equal, your rectangular box will be square.
Installing beadboard over clay tile?
Answers:    Your rectangle is racked. You obligation to adjust the side until your diagonals are equal. That will be square.

You can try to get the diagonal to hit 42' 7-1/2" (the target length of the diagonal), but it's easier of late to make the diagonals even. If one is 2" longer than the other, move your lines 1" to engineer them even. Good luck.