I can only make 2 gallons at the moment though.
ripen the milk to 90F then, after removing 1 gallon of water - pour 150F water in over the course of 10 minutes
Sam, there's the answer - the recipe you are following must be based on a larger make, probably 4 gallons. You need to remove whey as a ratio of the total milk volume - 1/4 of the total volume, if my guess on the make size is correct. Based on the two quotes above, it sounds like you removed 1/2 of the total volume, and for that ratio, it is not surprising that adding that much 150° water back brought the temperature too high. Try removing 1/2 gallon of whey next time (=1/4 of the 2 gallon volume), or, as John says, change the temperature of the water you add.
Always something to learn ...