Have read several posts that state that the manual j oversizes systems. Please tell me how they are oversized ?
In my world, it seems to undersize them.

I bought a ten year old home wih 1950 sq. ft. The system was a 2.5 ton HP. At 85 degrees or higher, would not bring the temp below 75, at 90'f' would not go below 78. It was installed per manual 'j', per the paperwork from the original homeowner.
In my experience, manual 'J' sizes for the 'new' concept.
once windows & doors have been opened and closed a few times, the seals are not the same. More infiltration. The
unit gets older & does not produce what it did when new.The combination causes the system to be short a little after about five years.
Have been on many calls where the system was put in by competitor, and worked well for a few years, but just doesn't seem to be keeping up now. Most of these, to me, was sized 1/2 ton short, at least.
Manual 'j' works well here for people who set their thermostats on 78 'f' in the summer. But if you are the next owner of this system & you like it to be 73 in the summer, forget it.
I do manual'j' calcs, but I allow for the other 'possibilities'