I just use a (spare) tape measure and convert the height of the liquid to volume on a spreadsheet (or in my head if needed).
Post-cool volume measurement is taken from the top of the wort to the top of the kettle (to minimize sanitized wort contact).
Keep the tape clean. If you have a rounded-bottom on your kettle (keggle) you'll have to account for that and always take the measurement in the same place.