building a helix from scratch?

[Sumpter250]

A 14" radius, gives you a circle circumference of 87.9", so each complete "loop" of you helix will take 88" of track. To rise 6", with 2" clearance will take three circles, minimum.
If you use quarter circle pieces (to get the most out of a sheet of plywood), you have to allow for the reinforcing cleat at each joint. This affects clearance, and, therefore, the grade of each circle of the helix. I would consider 3/8",or1/2" plywood, with edge doweling of each section joint.

And if this plan works the way I want it too (which I'm seriously reconsidering this part because of the complexity it will require) is a 20" helix with an off ramp @ "8 inches.

The only caveat here, is that your turnout for the off ramp be placed far enough back to allow the "ramp" to follow the helix, at a decreasing grade around the outside, to insure there's no sudden change of grade. Just as curves work better with easements, so do grade changes, and this will be happening under the layout where derailments, or other problems, will be particularly frustrating.