Those numbers refer to where the middle of your boot is in relation to either mid sole, core center or true center. Traditional mounting is at the mid sole mark, which is usually around 7cm back from dead center. Core center is the thickest part of the ski and is the same as true center in symmetrical skis. True center is just the absolute middle of a ski. For park riding, mounting closer to center is better for balance and even swing weight. For charging/racing/all mtn skiing, further back from center is usually preferred.
they're not bad actually. Dont forget that to hit a large kicker, you have to be doing a fair speed, so even though park skis are fatter than race skis, they're not much slower tbh
I'd still centre mount, an adjust your big mountain riding style to suit the ski, i.e. in the back seat a little to improve the float/ responsiveness.
Just out of curiousity,which skis are you using to do big mountain and urban?
the mad trix will limit you totally. A 75 waist will make it almost impossible to do big mountain, and i'd guess it would be fairly unstable on landings aswell. From what you've said, it sounds like you need something a bit fatter probably, something in the mid-80s would be betterSomething like the Salomon Suspect, or the Volkl Ledge are good all-rounds skis, and won't limit you
But if you want to do mostly urban/ rails, then something like the Amplid C7 or Line Invader might be better.