HCG - 60% rule

I see a lot of posts now where women say HCG should double every 48-72 hours and a lot of people say it has to go up 60% minimum in 48 hours (including my RE). I’m just confused (and maybe brainwashed from too much time on this app lol) because I always thought HCG had to truly double in 48 hours to be viable. Does it double in 48 MOST of the time and the 60% rule is more like 50/50 could be viable might not Be?