To be a preacher of any sort you need to go through trial services. In some places these are taken very seriously. In others those assessing says things like, 'ag, he really wants to be a preacher, we'd better pass him' and give an arbitrary pass mark. I thought about this very hard last year when I had to do a trial service to be accepted as a Methodist student minister. I got 60%. Others got 80%. Somehow I didn't think I was worth 80% and wondered what those others had! But I devised this scale, and it works for me. It doesn't really work for ministers who preach every Sunday and people have no choice but to listen to them!
90% - people will travel from far to come and hear - eg Angus Buchan?
80% - people will make an effort to hear you regularly
70% - people (who know you) do not stay away when you are preaching
60% - there's nothing really wrong with your preaching, but there's something missing
50% - there is a spark of potential or this is a dying gasp.
I reckon that I am a 70% preacher. If I lose energy I sink to 60%. I would love to become a 80% preacher!