Try to Google HVAC load calculations. There are a lot of factors that go into it. Measurements of outside walls, which direction do they face, window size and types. The insulation in the walls, square footage, etc. As far as your furnace converting to upflow you need to check with the mfg. The 100,000 btu sounds like it's in the ballpark,but the 5 ton seems like overkill. You want the a/c to run long enough to cool and dehumidify. With a 5 ton you will cool quickley and not dehumidify very well.