To gage how much wort is in my brew pot I have a large SS spoon that has gallon marks scribed on it. To make the marks I added one gallon of water at a time and put the spoon into the kettle until it just touched the water. I scribed a line on the spoon at the top of the kettle. I repeated this until I had 10 gallon marks on the spoon. It works great for me.
For mash and sparge water amounts I've always used the charts in Papazian's The New Complete Joy of Home Brewing
as well as ProMash for guidance.
If you use a quart to 1.25 quarts of water for the mash and 1/2 gallon for the sparge per pound of grain you should be fine. Remember that the more grain you have the more wort ends up in the pot. High gravity brews require more boiling.
I fly sparge and I keep the water level above the grains until the water runs out. I then let the water level drop until flow stops or the gravity drops below 1.012. (On my system it usually stops at about 1.018 or 1.020.)
Keep plugging away! It sound like you have learned a lot from your first AG. It does get easier.