So, here are the critical steps to getting your water volumes at least close to target:
First, if your vessels are already marked with volume scales, disregard them until proven to be accurate.
Next, take a dip stick and a gallon or liter measuring cup and a dip stick you can mark. You can then measure in volumes based upon most reasonable to measure volume which is applicable to your measuring capability by adding in a gallon or liter at a time and mark/measure the height corresponding to the amount added into your vessel/mash tun/brew kettle (easiest and most useful is to do this by amount added so as to come up with a scale appropriate for that vessel.)
Third, if you are draining through a spigot at or near the bottom of the vessel, drain the water out as you would if draining the wort after the mash. Measure the volume which you can drain out. The difference in volume in versus volume you collect draining out is your volume loss to dead space.
Add the water you drained out back into your kettle (or add a known amount to the boil kettle) and turn the heat on. Allow it to come to a boil and boil for 60 minutes. Measure the amount of water you have left. The difference in water going in and water after boiling is your hourly boil off.
Once you have these figures, update your equipment profile using the mash tun dead space and boil off rate you just measured. Remove any top off water you have to the boil kettle and/or fermenter.
Save this profile and update a copy of your recipe with this new profile. Now check for mash tun volume needed on the mash tab. If this exceeds the volume (water plus grain displacement), either adjust your water to grain ratio in the mash profile to allow it to fit or go back to your equipment profile and add the amount of top off water equal to slightly greater than the overage from the total mash volume required to either boil kettle top off or, if the pre-boil volume would be greater than the boil kettle would hold, to the fermenter top off.
When I do this, I usually target my volume into fermenter to a value of the batch size plus trub loss initially until I get an amount which covers trub loss which covers most or all of the trub which may occur with heavy hop loading. Once you are comfortable with that, you can set the trub loss to that number and reset the batch size to the desired amount into the fermenter. Remember that each time you make a change to mash profile or equipment profile, you will need to update the profiles in each recipe you want to apply it to. I use the date that I update the equipment profile as an immediately identifiable reminder which profile I have set up the recipe.