The 225 Winchester is a rimmed case. When you cut the chamber, you will also cut the rim relief. Run the reamer in until you cut the rim depth to .001" - .002" deeper than the rim thickness. The rim thickness is .049" thick so cut the rim depth to .050". You will be head spaced properly. the other way is to buy Go gage and No Go gage for the 225 Winchester. As you get close to the proper depth, but still short, drop the go gage into cleaned out chamber to see if it will close on it with the barrel on the action. Take it back off and keep going deeper and refitting until the action will close with the go gage in the chamber. Once you do that, check the no go gage to ensure the action will not close, if so, you have it properly head spaced.
To be sure on rim thickness, buy your brass ahead of time and measure the rim thickness. It should be .049" thick. Also, the shorter you cut the chamber, the less case stretch you will have, so you are better off to have the chamber just deep enough to close your action on and no deeper. Remember, if you cut it too shallow, you can redo and cut it deeper, once you cut it too deep there is nothing you can do, except cut a chamber for a bigger cartridge. So be very cautious when approaching the proper depth.
Good Luck and Good ShootingSorry, After rereading your post..... I thought you wanted to know how to cut a chamber, not make head space gages. If you use the first method above, you will not need head space gages.
Good Luck and Good Shooting