If it's really a campaign that is intended to run for very long, then I would work with pre-defined ranges, where the BTC/post is already pre-set. No questions, no discussions, no manual decrease/increase, everyone involved knows what they are up to, what they will get paid or what they will have to pay in case BTC is yoyoing during the course of the campaign.
Example:
Just round the BTC rates to make it look more tidy.
You can add more/smaller ranges, especially if BTC goes sub 10k (you never know....).
You can make the $ amount higher when BTC goes south and lower when BTC goes north (people will be happy anyway if BTC goes up, especially if they didn't spend their campaign earnings yet ).
Can easily work out some scenarios with Excel and some formulas and play around to see what you and the company you work with like the most. Then post the table with the payrates in the first post of your ANN et voilá.