I would dedicate a ruler or make lines on a nylon spoon or use some method to gauge your water precisely. Use a bottle with a known specific volume to incrementally fill up your mash tun and boil kettle (unless they are virtually the same type of vessel - then just do it for one of them), going say, a gallon at a time and marking it on your ruler or spoon or stick or whatever, so that you can have a reasonably good gauge of volumes. This will allow you to know your volumes as you go and allow for differing boil off rates given differing humidity levels, etc.... I have found this to be invaluable even now after many, many batches. Once you get things dialed in, you will hit specific marks based on grain amounts almost without thinking.
Cheers! and welcome to the forum. Read Denny's bottom line for what most people think about this forum - a great place.