Let's walk through this. The formula for determining the circumference of a circle is 2X pi X radius or pi X diameter where pi is 3.1416.
A 2% grade is about 1/4 inch per foot so 350 inches of run is correct.
So if we divide 350 inches by pi (3.1416), you get 111.4 inches or 9.28 feet. That's for one turn on the helix. To get 2 turns obviously divide by 2 and get 55 inches. Your math is correct .
The situation might arise when you are actually building the helix as you might not have enough room between the turns. You also need to account for the baseboard that the tracks will sit on. Also you will need access space for the "0-5-0" wrecker in case of a derailment .
I built a layout long ago with a 78 inch diameter helix that climbed 21 inches . It took 3 turns with a spacing of 7 inch per turn but there was only 5 inches of space between the rail top and the bottom of the baseboard.
Good luck with your build.
