If they are experienced, you should take into consideration their feedback on what they have been doing. Most people in my opinion could get away with doing less volume than they are doing. You can usually almost immediately lower the volume and up their intensity if it's not already high. Even higher volume guys won't lose much if anything if you back them way down. So I always err on the low side and essentially "see what you can get away with". Monitor feedback, and increase if / when necessary.