You're welcome, bandier. The stock offset on the 20x10 1LE fronts is +20, so you're 5mm off, but you can use 3 or 5mm spacers to adjust that back, so I don't think you'll have an issue.
I'm obviously just a guy and not a wheel/tire/suspension tester, so I can't be 100% sure, if only because my car has magnetic ride control shocks and yours has the base suspension, but I did try many combinations over the years, and the spacers will give you some leeway no matter what.
Relax, this is nowhere near as bad as you think.
Your new 285/35/20 fronts will be +0.7% (ie taller), your 305/35/20 rears are +2.5% (also taller) compared to the stock 245/40/20 overall height, so you're changing the ratio much less as you're increasing the height on both ends. As you quoted, my napkin calculation here is simply
delta = rear - front using the proper sign, so your ratio change is only ~1.8%, which will be just fine.
Your mistake is in thinking these quantities are always additive, but they are actually directional, so to get the actual diff you need to subtract. If you inverted the directions between front and rear (say, taller fronts, shorter rears or vice versa), then you'd have to add their absolute values (magnitudes), but that's not what you are planning. Hope this makes sense.
The rake change is not super significant, but you would probably feel it as increased lightness in the front when pushing the car, which you don't want.
My choice for a non-1LE like yours and mine would be 275/35/20 and 315/30/20, this will work fine with zero issues, no tuning required. This is the setup I actually purchased last year when the time came to get new tires, and it's just perfect. The only problem is the limited availability of this infrequently used rear size, but as I said, everything's a tradeoff
