Your helix has radius 200 and the tubing as radius 30. Correct?. This conflicts with R which says they're both 200.

In this event R+r=230. That's what I'll use.

Assume you're wrapping a tube of radius 30 around a helix of radius 200,

This forms a helix. A helix can be represented by

a in this case is R+r=230. The radius of the helix + radius of tubing.

Since you want two revolutions when z=500 units, you have

You have the helix:

It's length in making two revs in a distance of 500 units is given by

Now, this is a cylinder of radius 30 with height 1466.6

Check my calculations out.