Just remember that this is the "Tops on removing a tyre" thread .. I know it's tempting at this point, but no matter how many times you migth want to measure it, I'd advise against cutting it.I am a big believer in "measure twice , cut once" so I had checked and double checked, honest I really did!!
You also should know that you're going about all this wrong. The reason you're struggling is that you're trying to remove the tyre from the wheel. It's far easier if you remove the wheel from the tyre.......