Just stick to changing one of the values, the rest of the width/depth values will change proportionately. Just keep trying, there honestly isn't a "one size fits all" method to scale anything in pepakura. Try to measure your head to get a rough idea of what size you should be looking at, and of course add a few cm for room to put padding and whatever else inside.
If you feel like you're wasting your time with wrongly scaled pieces, don't. You're getting in good pep practice, something all of us could use. Heck, I built 4 Mark VI boots before I found the scale I liked.