Carton divert

Using a PLC5/80
I have a conveyor system that has one line that diverts up to three levels. I would like the cartons to go to one level depending on the amount of cartons on the single infeed conveyor.
I could do this simply by calculating cartons per minute or amount of gap between cartons, I'm not sure how accurate this would be due to upstream conveyor issues. (ie. carton jams etc.)
I would like to check the gap every 10 secs then average the value after 1 minute. This would determine if another level should become available.
Am I making this way to difficult?
Thanks in advance.

I guess that I should have been more specific.
I have a slat shoe sorter that will divert cartons to a conveyor depending on lane assignment from a overhead scanner.
On this divert is a photo cell (retro reflective: transmiter p/e and reflector) that only determimes divert confirmed or if a jam condition has occured. This is the p/e that I would use to count cartons or gap between cartons
The conveyor that these cartons have the option to divert to up to three other diverts or levels that will recirculate unless an associate puts the cartons on a rack. I guess my question is what is the best way that will determine if cartons should go to one level or open up another level depending on the amount of cartons.
I could count the gap or number of boxes and use a GRT than or Les intruction to do this but I thought that someone would have a better solution. This would only enable an output for a swivel wheel divert and not turn on or off the conveyor system.