How to calculate central AC size

I'm in the process of getting quotes for a new central AC in South Florida, 3 quotes so far with one more scheduled. My Current unit is 4 ton and my home is 1,800 sqf with 8 foot ceilings, and new blown insulation in the attic. None of the sales reps bother to spend any time examining my home, no walk through, no measuring of rooms, no inspection of current duct work... one would think there would be a bit of science involved. So far they just want to put the same size as currently there (I did not install the current unit and have no clue what led to the decision of it's size). I understand there is anywhere from rules of thumb to a heat load calculation, to the manual J calculation... rules of thumb seem to prevail in my area. The issue I'm running into is with every calculator I find I need a 3.5 ton (ex 1800/500 = 3.6). Long story short, what is the best way to calculate the size for a south Florida home? Which is worse, undersized where the risk is inability to cool my home, or oversized where the risk is short cycling and higher energy bills. All this feeds into a second issue I have, which is the current single return grill is 20x20, which I understand is too small for a 3.5 and 4 ton unit. I plan on increasing the size but need to know what ton unit I will purchase first such that I can select the correct grill size.

source https://hvac-talk.com/vbb/threads/2239455-How-to-calculate-central-AC-size?goto=newpost