With new stats aimed at "Comfort", you will not always get run times over 10 minutes during the milder days.
A 10 on 5 off, is a 4 CPH setting on a CPH stat. I prefer 3 CPH. Gets you either a longer on time, or a longer off time. Or combination of both.
I don't undersize A/Cs, if its a border line 2, or 2.5 ton. I install 2.5 ton.
I can slow the blower down to drop capacity in that case much easier then increasing cap on the smaller unit.
It was never recommended to adjust or verify charge without letting the unit run for a min 15 minutes. Just a bad habit that many people did, and still try to do.
If its 70* outside, and a customers A/C runs for 10 minutes to maintain a set temp 70* indoors, that is a long run time.
I have never had a customer complain that the system held 72* when it was 95* outside. But customers do complain if the temp goes up to 78* when its 90* outside.
You may undersize them to meet your personal efficiency requirements.
I will size and install them to meet customer comfort requirements, After all. I sold them a system to make them comfortable. Not to meet my expectations of comfort or efficiency run times.
That is why its important to offer choices of system types. (single or 2 stage, PSC or VS blower)
Its like buying a car to drive long distances. You consider both MPG, and comfort of ride.