Go to home depot/lowes and get a long piece of skinny PVC pipe.. I have one that's like 6 feet tall by maybe 1/4" in diameter... get a one gallon pitcher that is graduated. Fill your brew kettle with a gallon of water, put your "measuring stick" on the bottom of the brew kettle, and mark it with a sharpie. Do this over and over until you get to 5, 5.5, 6, 6.5 gallons. Make sure you indicate which one is 5 and 6 with a special marking. While boiling keep the stick in the bucket with the star san so it's ready to go. With about 15 mins to go in the boil drop the stick to the bottom of the kettle and see how much water you have, add the appropriate amount, careful not to add so much so fast you lose your boil. get it up to 5 gallons and you are g2g.
It sounds like your OG was spot on if you were to actually have had 5 gallons. My last stout was over a smidge too, but I had 4.8 gallons... this was before I knew you could just add water. Also when you boil keep the heat pretty low, don't do a raging boil. Be consistent in your boil temps and make a note of exactly how much liquid boils off in the hour. Probably in the area of 1gal per hour depending on temp and width of your kettle. Then when sparging make sure you have 5 gallons PLUS whatever your 1 hour boiloff amt is, which will probably equal 6 gallons or a bit more. Good luck, and congrats!