Hi hutch42,
All E350 models stand 3" lower than E450 models. We own an E350 so I subtracted 3", working with 97" instead of 100".
The 97" mark is approximately just above the B+ cap front-facing amber driving lights or about 6.0 feet to the front bumper. That is as good as I can approximate. To play it safe, I would subtract a half foot making it 5-1/2 feet of PC you can park under your deck.
The diagram you sketched out and your estimate is quite accurate. Figure on 5-1/2 feet.
When it comes time, I would anchor something like
THIS into the driveway to bump your front tires against to stop your PC at just the right place to get you consistently comfortably close to the deck without making contact to it.
Be mentally prepared to watch squirrels jump to-&-fro between deck and PC.
Ron Dittmer