all personal preference in the end. try different positions and see what works best, typically if you ski switch a lot you'll want it more upright (among other reasons... and vice versa for big mtn skiing) but everyone is different.
SLOPE-STYLE. FREESKI HARDGOODS & APPAREL. BRECKENRIDGE, CO
Well if you are talking about forward lean, most boots now are pretty upright any way. Gone are the days were you had super aggressive forward leans.
But in regards to how much will work for you it's more dependent on your build and body type. For example a guy who is 6' is going to have his knee far further forward in a boot then a guy of 5'. So it's not so easy as saying you need this for this type of skiing ect. But luckily that why boot fitters exist. They can help get you in the correct style if boot for you and also your riding style