You are right Dave,it is possible to know exactly how long it takes you to do your stage set.I time mine to the minute add on a bit of chat and you have an accurate time for your spot.All it means is that you have to run through what you are doing for your act during your practice, time it and then you know exactly where you are.These artists that have no idea of their act timing are a major problem and well prepared artists suffer a sudden reduction in their allotted time.Sometimes, like us, it happens just as you are going on stage and it does throw you,what do you leave in? What do you take out? All this os happening on stage.I suppose proffessional artists can cope with this, but it does create a lot of difficulties which are not of your making.
Al