Have come across some different ways of quoting ones average.
1. The number of full size hives at the start of the season come spring after accounting for winter losses, DLQ etc. So if I start the season with 10 and end up with 15 its the 10 I use to calculate my average.
2. The numbers of full size hives you had at the end of the season.
Both options have been used by presenters at meeting, sometimes I have only found out after asking.
I can see advantages and disadvantages of both. Option 1 allows for splits which may both produce some honey but perhaps no more than if the colony wasn't split. It also allows one to 'inflate ' ones average yield.
Increased stock from collecting swarms and moving nucs to full size hives can generate significant amounts of honey but aren't accounted for in option 1.