Hmmm. Why would a 7Ah require more life than a 4 Ah to be considered good? I would think that either (a) both would require the same ( just for the sake of keeping it simple) or (b) the 7 Ah could accept a lower reading because of it's larger capacity it could sustain a drain of a specific amount of current for longer period of time.
Or that the current draw and amount of perscribed standby requirement for the system the battery was being used in ..... over what period of time .... would be thrown into the calculations.
Not just a preset number. (?????)
Regardless where you got the reading ..... it would be wrong.
Even with the recent increase in battery cost, I'm of a mind that it's better to change a battery, if it's been in for a few years, even if it reads good. I don't feel I'm just "selling them another battery". I'm taking precautions that surely reduce the possibility of a battery failure. It's not a "rip off" That's what they pay me for ..... to anticipate and head off potential problems. I'm sure they'd rather do it that way then wait until they get a low battery signal and have to pay me for a service call just to change a battery that I could have changed a year ago .... the last time I was there.
When Yuasa, first came on the scene, finally providing the trade with reliable rechargeable batteries, the method I use was recommended by them. Since I do primarily residential now, and in the event that all else fails, the newer alarm systems provide a low battery alert, there's no reason for me to have to document a battery condition. So I just continue to go with what's worked for so many years. (Other than a burned finger every once in a while)