How did you make that calculator? Consulted/interviewed a thousand sailors? ;-)
I think there are some adjustment to make as Mike says. For example: you could use a 7.3 sail on an iSonic96 (59 wide) and then jump 3 sizes to iSonic122 (75 wide) with the same sail. Obviously you'll need a different fin.
That said I think it's an excellent, if a bit rough, ws gear guide. For someone without enough experience with different gear, it will sort them out.