To the OP I am sure you are trying to be through and meticulous in your approach but in this conversation you are being very obtuse. You have been told several times that based on your own word this current system maintains temps way beyond design. If your current system will maintain greater than TWENTY DEGREES indoor to outdoor differential and cycle on a degree day IT IS OVERSIZED. A properly sized and designed system SHOULD RUNS CONSTANTLY at or over a design degree day. So in New York lets assume a design degree day is 92* and you can maintain or reach 70* your unit IS NOT UNDERSIZED. If it runs constantly at 92* to maintain 72* then you have a properly sized unit.
I design for Huston and I design for 75* 50% RH on a 100* day and I do homes over 4000 sq.ft. with under three tons all of the time. If you have an area of your home that is uncomfortable but you can maintain the temps you state YOU HAVE DISTRIBUTION problems not a size issue.
Just for your edification I do not provide load calculations or material list until a contract is signed. I have had numerous Homeowners take my load calcs and design and have other companies perform the work for less money. They all call back because it does not work because the other company does not have the skills or knowledge to properly install the system. I have even had a few with the temerity to demand that I fix the problem at my cost because I did the design work.
At this point I would recommend going with the original contractor or find a new contractor DO NOT GO WITH THE FIRST COMPANY they are a dog and pony show contractor.