It's been hard for me to judge this aspect. I'll have customers call and overestimate how many will want their faces painted or underestimate, so I've taken to carrying my quick board and my more elaborate board in my car to my events.
I do a Chick Fil A event that I can never judge accurately. I took my elaborate board the first time I went, because I had one artist say that it was pretty low key when she went. I had a long line the entire night and everyone seemed to want the most time consuming designs. Last week, I was doing the same Chick Fil A and I had some people from another restaurant saying they were interested in hiring us and were coming out to see what we do. I took my quick board and my book with more elaborate designs (just in case) and ended up being sllooooww the whole night.
For some reason, the management hadn't advertised on the sign outside about face painting and we ended up being slow, so I'm sitting there with a lot of down time. And the people that were there, seemed to choose the quickest designs and when I got them to browse through my book, hoping they'd pick something more elaborate, they found the cheek art at the back and were picking things like a baseball on the cheek.

Needless to say, I haven't heard back from that other restaurant.
Back to the topic of the discrepancies in fast or slow designs. If I find I'm running low on time with too many faces left to paint, I direct them to the fast board and let them know I can only paint designs from it. I also have a small sign that I put on my table when I'm really busy, telling customers they need to have their design picked out before taking a seat. That can be such a time waster, when people don't know what they'd like yet.