Good Topic.
I recall Robert Saunders teaching us about the cost of carpet cleaning, of course we all thinks it's free to us as it's our time a few pence on chems..
The best way to work it out is say your yearly espenses were £15k divide that by lets say you do 350 jobs a year.
= £42 per job.
So £42 is you break even on every job, with that information I would say min order should be around £75 +
I know that does not quite answer your question.. but it's all I have