Like someone else mentioned...don't cut the back section off right away. What I do is draw my lines...match the corner of the square to the corner of the rectangle...pin...one pin right in the center of the line...check to be sure the corner of the square is still exactly meeting at the corner of the rectangle. Sew just on the line, using 50 or 60 wt thread (I like serger thread, though it says it's 50 wt..it's much thinner...and stronger than sewing thread).
Now...instead of cutting, fold out the square so the inside corner matches the outside corner of the rectangle. PRESS..you have to press.
Do the same for the other side...and yes, this square is going to overlap the first stitching line...it's supposed to.
Once you press out the 2nd side same as you did the first...check the point...where the squares meet at the center of the rectangle...you should have about 1/4" seam allowance before the point starts.
Now is when I open out the square...sew 1/2" from the first seam line...and cut between the seams...this gives you a small half square bonus triangle from each side of the rectangle...you could just trim the seam...but I try to avoidwaste where I can.
Now, you want to check the size...if it's a hair too large...trim from the right left and bottom...never from the point. If it's less than 1/8" short...I adjust by stitching right next to the original seams...then I remove the original stitching. Once you have your flying geese pieces & want to sew them to a strip or another block...your point is your stitching guide...sew just inside the seam allowance...almost touching where the seams formed the point...but never over the x seam that forms the point.... X is the seam...top of the X is the seam allowance, bottom is the flying geese section...always stay just a hair above the center of the X...and your points will aways be right.
Another thing I do...if a direction asks for a measurement of 4-1/8"...I cut 4-1/4"...if it asks for 4-7/8"...I cut 5"...it's easier to trim a block down, than it is to make it larger by having to shorten the seam allowance.
Hope this helps.