Now that I have had a chance to test it in the lab, I have an answer:

First, I used an 18F8723 with

OSCCON = $62 ; 4Mhz, INT osc.
OSCTUNE.6 = 0 ; PLL disabled

Adjusting OSCTUNE[4:0] from %01111 to %10000 changed the
frequency from 4.507 Mhz to 3.440 Mhz.

The change was nearly linear. With each count giving about .8% frequency change.