Calculating the temperature for you infusion mash can be a little tricky.  It seemed like I was always just adding hot water and trying to get a temp. then adding more until my temp. was were i wanted it.  This did not produce a very reproducible method for brewing beer.   Brewer’s friends mash calculator has made this much easier, and even allowing you to create a multi-step mash schedule.  If you missed your temp they also have a quick version that will allow you to see how much  boiling water you need to raise the mash temp to a specific target temp.