The spray starching will help alot with the bias plus remember when you do go to connect this pinwheel to the next block you might have to stretch just a little or ease up a little to make corners or points to match better - but that happens on most everyones. Also try to make sure that you are butting up your seams and locking them together it sure makes the block lay smoother.
Thanks,
Sherryl
Candlequilter