Make yourself a dowel marked w/ volumes by the gallon using hot water. This will give you a means of measuring your pre & post boil volumes.
I use a square dowel and use different sides (& colors) for various kettles.
You also need to determine your boil off rate to have better control of your process. Fill your BK w/ 5-6 gal of hot water using your measuring stick. Boil (rolling) for an hour & then measure your volume again. The difference is your boil off rate. Keep in mind this can vary w/ humidity, etc. Use this info to set up your equipment profile in whatever your brew software of choice might be.
Last, keep a few gallon bottles of spring water on hand to adjust volumes as needed.