Hmm, I think you have a miscalculation about neutronics here, and fission/capture ratios. Cf neutrons are in their majority come nicely in 1MeV-2MeV, so you have much more fission going per neutron, and much less thermal neutron captures.

If you have an intense neutron emitter dispersed in fuel + moderation + reflectors you should be able to go with much lower k to generate a meaningful amount of power.

You should me able to go down to single grams of Cf per megawatt. Yes, not that economical, but at least possible for things like reactors for space use and such. At that price, that will be cheaper than current RTG material.

Correct me if I am wrong.