Below the belly button, but since the fighters are waering trunks it's to the ref. call......
Unless Majesty has see through vizion and can see the punches landed below the belly button, then the only other way would be to ask Majesty exactly where he considers a punch to below.....

Sometimes the ref. signals dead center in the top of the trunks where they put their names, now if you notice some fighters belt line is thicker others it's thinner, some wear the trunks high to give them less of an area to hit and a bigger area for the ref. to call a low blow....