Hate to say it gents, but I feel that you're looking at the wrong variable.

In order to maintain the original staggered wheel proportions, you shouldn't necessarily be focusing on the relative sidewall heights, but moreso on the overall tire diameters. When looking to fill out the wheel well in the same staggered proportion as the OEM tires, it's the diameter that counts.

So if you change the front tire to, say, 235/40R18, you want to look for a rear tire that is taller by the closest overall amount so that the wheel wells are filled out proportionately. Use the fitment guide link above and do the work with a little spreadsheet. You'll find that running a 30 aspect ratio in the rear fails to maintain proportion.

I'd show you the results, but doing it yourself might be the only acceptable demonstration. Does that make sense? The best solution is 275/35R19.