Boil mess up

So, i thought I was following the directions to the T. Being my first attempt I am not good at improvising yet. It took the water quite a bit of time to boil after I had steeped my grain, but once it began I was careful to time the 45 min boil as per the instructions. I had closer to a half gallon when all was said than done as opposed to the gallon I was supposed to get. So obviously it boiled down too much/ What are ways to control how much boils off? I can only think of messing with the temperature ensuring I have minimum heat required to attain boil?

You can try adding a half gallon extra before you start and you will boil that off leaving you with what you need.

Add more water to start or top off when you’re done.

You can also keep the pot lid partially covering the pot. Not full on but balancing part on/part off.