Technically, it's not the highest temp but rather the temp at which fermentation finished. Usually those are so close that it doesn't matter. But in theory you could ferment a lager with a robust yeast like 34/70 at 50F, bring it up to 68F for a short D-rest, then drop it down to 46F to finish fermenting. I doubt anyone would do that but, if you did, you would use 46F for priming calculation.