Another tires & wheels calculation question

Current tires are almost up so I'm thinking of dressing up my 360 Spider w/ a set of new wheels/tires.

Question is: stock sizes are 18x7.5 for front and 18x10 for rear w/ 215/45/18 & 275/40/18 but looking at other sizes that people put on I'm a bit confused.

By my calculation, if I move up to 19", in order to maintain EXACT same overall diameter and ride height, I got the right sizes to be 240/35/19 and 275/35/19 or 325/30/19? Obviously there's no 240 sizes so I guess one would settle for either 235 or 245.

What really throws me off are the sizes of rear tires that people put on:

285/35/19 (this one is close to original size!)
285/30/20 (this one is even closer!)
295/30/19! Abt. 20mm short in overall diameter
355/25/19! Also abt. 20mm short in overall diameter

So is there anything obvious that I'm missing in my calculation? Or is this generally within tolerance?

 

Or is this the wrong forum to post?