I would try measuring 5 gallons of liquid and put it in the water jug to see where the level is.
If you make 5 gallon batches, this is where your liquid will be when you put it in there.
Unfortunately, I think it will be a little too close to the top, and you will have more blowoff, since you have less head room.
Another possibility, is to make about 4.75 or 4.5 gallon batches, which leave you with more head room to work with.
I am going to run into the same issue, as I am going to use 5 gallon buckets, as they were free. I will probably make the batches 4.5 gallons of liquid!