I am guessing you are making an 8 point "sawtooth" star. The most common reason for point cut offs is seam allowances not being uniform. What I was taught to do is check every allowance as you go along until they are regularly right. I generally make my sawtooths with HST blocks, although it is just as easy beginning with flying geese. I cut my HST's 1" larger than the finished size I want. This gives me 1/8" for trimming, after I press and use starch or sizing. When I use the square over a rectangle method, I need to remember to be 1 thread width off the center line with my sewing. Pressing is very important here too.