This may be repetitive, but I didn't see it...........as you're doing what others mentioned, mark a dowel or rod with the gallons as you fill it up. Part of accurately calculating the losses along the way is knowing where you started.
Mark a stick with the typical pre-boil level, measure what went into fermenter, what would not drain out of MLT and boiler, how much you put into keg or bottles, etc. After a few batches you will know the losses, and have a good idea of what to start with IOT finish with your desired beer QTY when it's over.