Give this spreadsheet a try. (Linked from the Crossfire Document collection)

I have 245/35R18's on the front. With a sidewall profile of 35 on a 20", you are going to be WAY too tall. Just over 1.6" overall wheel height, raising your vehicle by over 3/4". A 245/30R20 will be about 3/4 of an inch taller overall than stock height, raising your vehicle in the front by about 3/8". A 245/25R20 would be 1/4 inch shorter overall, lowering your vehicle by 1/8".

The offset calculations are in the spreadsheet. I have no rubbing at all with my 245/35R18 tires on the rims listed in the spreadsheet. No spacers in the front and 32mm offset.

Also, keep in mind your rim width vs. the tire width. There is a maximum tire width you can put on a rim before it will break the bead.

Hope that helps.