I've found that the "plus outcall fee" just gets too confusing when there are more than a couple of people.
For example, if your outcall fee is $20 and 11 people are getting a massage, each person is meant to pay how much? One of them got an hour massage one got a 15min massage and the others were a mix of 30min and 45min - do you still plit the outcall fee evenly or pro-rata between lengths of session? Way too confusing for me to figure out on the day since they say 14 people and only 11 end up getting a treatment. ANd who do you approach about the outcall fee? Gets confusing....
So I just put the outcall fee on the hourly rate, say your hourly rate is $60 with $20 outcall fee, for parties, it's $80, no outcall fee. Don't forget you've taking a lot of extra linen, particularly for the 15-20min ones - it's still the same set-up time for each 15-20min session as a one hour massage - that's 2 or 3 extra setups you need to factor in somewhere, having the extra cushioning of the outcall fee factored in to each hour means you don't have to worry about it.
If you get 6 hours work out of it and feel you're doing a bit too well with the outcall fee inflated hourly rate, give the organiser a free massage or half price or something - I'll usually do half price once they go over a 4hr quote, free once they go over an 8hr quote, but I don't promote that, they agree to the full priced quote and get a nice surprise on the day. If they turn into a complete bunch of pains where they take 20min each to get on the table once I've asked to come in, or they've not filled out the intake form before I am ready for them, etc, I don't make the offer, but I don't feel too upset since I know I've got the bit extra factored into the rate. And after 2 party guests like that, I will make an announcement so the rest get a bit better organised.
Or if one person is paying for you to be there, they usually organise everyone. It's just if the individuals are paying for a 15-20min massage, they expect 15-20 on the table, because it you include get on the table and get off in the 15min, they're only on for 5min and feel shortchanged, so you can't say "your time starts now" and then they finish their conversation, go to the bathroom and you have started the next person because their time is up.....