You're right about DC resistance simply being a rough guide to impedance. It's not a strictly linear relationship, though. I'll try not to go into too much depth, so bear in mind there are a lot of factors I'm not going to touch on, for the sake of simplicity. I'm happy to get into the nitty gritty if you're really interested, though.
Very loosely speaking, impedance is the sum of the inductance of the coil and the DC resistance. The inductance is increased by increasing the turns of the coil, and by keeping the coils tightly packed.
The DC resistance is basically decided by three factors: the material the wire's made of, the wire gauge, and the length of the wire.
Assuming the first two factors are always the same, a doubling in the length of the wire will double DC resistance, and will have a predictable effect on impedance. Because you have a longer wire, you have to put more turns on the bobbin, which means you'll increase inductance, and thereby increase impedance. Assuming machine-accuracy, this part can be nearly linear...
However, BKPs are scatterwound, which means although the number of turns will have (roughly) doubled, the inductance wont increase in a linear fashion because the coils wont be exactly side-by-side, hence, not as tightly packed as they could be.
Keep in mind, too, that resistance can be changed by changing the wire gauge and material, without effecting a change in induction. This will still affect impedance (because impedance is the "sum" of induction and resistance), but it wont have the double-whammy effect. So you can see the relationship necessarily can't be linear.
I hope that's not too confusing, and helps answer your question. I've used some terms very loosely in order to not make it any more confusing than it needs to be, but let me know if you want clarification or anything :)
-Liam
