That is basically the same amount of wiggle room i have on my forearms as well. If you end up wanting more space, you can always trim the return edge off later. Always do tape tests like you're doing, and dont be afraid to give yourself a little extra room if you think you'll need it, because even if you glue the fronts of your armor first, you still have the back sides to trim material off of. Check this thread, it looks like you have ANOVOS?
http://www.whitearmor.net/forum/topic/35086-ukswraths-anovos-tk-build/page-10#entry461326