I personally wouldn't advise a grade of 4.2%. I think 2% or even 2.5% is ok, but any steeper and you're asking for trouble.
Your math is mostly right, but there's one thing you didn't think of (and I don't believe any of the others who responded pointed this out)...
If the two loops of the helix are STACKED, i.e., on top of one another, or even cross over one another, then they will probably have to have a minimum vertical separation of at least 5 1/2" for the train to pass under the next level of the helix! So for practical purposes, you would have to gain all 7" in just ONE loop. To keep the grade manageable, that loop would have to be about 9' across. That's a floor area of at least 9 x 9 feet. You didn't share the size or any details of your furnace room.
Luckily there's another option. Using your current minimum diameter of O54, check out this crude sketch of what I called a "whale tail" :
It gains length by folding back on itself. I think you can get your 350" of run in a smaller 6 x 10 footprint. Note, those reverse curves are going to add a lot of drag. Personally I'm not a fan of die-cast cars. If your trains aren't made up carefully with all heavy cars at the front, you might experience stringlining. (Don't feel bad, the wrecks at Pennsylvania's Horseshoe Curve this year prove that it happens on real railroads too!) That's one of the reasons I don't advise going steeper than 2.5%.
Sorry for the crude sketch. And I welcome any additional feedback or criticism that would help Mr. JPS get what he needs.
