To measure the volume on my brewpot I just used a wooden dowel which I marked with a pencil. Nothing fancy but it gets the job done with reasonable accuracy and consistency.
Also I'd recommend keeping track as best you can all your volume measurements for a while. This will help you better understand how your system operates, which is really what it all comes down to. Once you get used to things you'll find you have to think about it alot less to get the desired results.