Measuring for Backspace Front Wheels

An 18x8.5" with 5.7" of backspace should fit just fine, especially with a 245 on it. Measuring backspace for the front gets complicated because of the turing radius, as zhandful mentioned the best way to do it is to see what others are running. Which is why I keep a spreadsheet of all the wheel/tire combo's I see on here, especially the ones that are running 17's and 18's and wider tires. 18's clear the tie rod unless you have drop spindles, and an 8.5" rim with 5.7" of backspace should clear the fender just fine.

Your tire selection isn't going to work though unless that's a typo. A 245/50/18 would be 27.6" tall, and there's no way you'll get those on the front, they'll hit the corner of the fender and then some. Even in the back that'd be pretty close to the limit. Not to mention you won't find that size for a car tire, they'd all be for light truck/SUV use.

For the front, 26" is about as tall as you want the tire to be. In a 245 the closest you'll get is a 245/40/18, which is about 25.6" tall (same height as the 275/40/17's on my Challenger). The other way to go would be to get a 255/40/18, which would be exactly 26" tall.