29c is quite an unusual size for a tyre.
I'd estimate the tyre size to be around 2140 - the size for a 700 x 28c tyre is 2136; next size up is 700 x 32c which is 2155.

Leaving it at 2085 will leave you short on each pedal revolution.

If you want a truly accurate measure, you'll need to actually physically measure the distance covered by each revolution.
Sheldon Brown has a guide on how to do this on his site:
http://www.sheldonbrown.com/cyclecom...n.html#rollout