I figured the fix offered by Sewbizgirl was what you needed. My fix was for when you figure the triangle between points wrong and don't have enough there.
Look at the back and your existing seam and cut units. If you need to, put a pencil dot where you want the end point to be and simply choose a slight angle from some part of your existing seam line. You can draw that or eyeball it. Test that angle isn't too sharp to make a distortion, simple test by putting a pin along the line and opening right sides to see if there is a pucker or other issue.
I usually press my seams open, but this is a place where pressing to the side really makes life easy
Back in the old days (pre rotary) quilters drew on fabric a lot. The choice was whether you drew the outline of your piece including the seam allowance, or did you trace around the finished size and cut the seam allowance outside of that. The advantage of using the finished size was that you could use the pencil line for your seam even with hand sewing.