How to measure offset: http://www.rsracing.com/tech-wheel.html
The MKs have about a +40 mm offset. I say about because the 16, 17 & 18" wheels vary. Keep in mind that 5 mm = 0.2" or not much.
We know that a tread width of 235 fits and in some cases even a 245 will fit. Why do some work and not others"? It relates to the bulge of the sidewall which relates to the width of the rim and the tread width of the tire. Tires are made to cover a certain rim width and have a normal appearing 'bulge' in the sidewall. If you have a fat tire on a skinny rim then you will have a large bulge or more curving out on the sidewall and that may hit on something like the brake line but the bracket for that can be bent to gain some clearance if needed. There is no substitute for a trial fit unless you know that someone has tried your exact combination and said that it works. There are no reasons that the tires sizes that we have listed would not work.
The aspect ratio is not the bulge. I know of no way to calculate it either.
Just a note here. I just dropped down from 235/60R18 to 235/65R16 and I'm getting better actual mpg. maybe 1/2 mpg different.