Assigning 5% of block space based on bitcoin-days destroyed (BDD) and the other 95% based on fees seems like a rather awkward approach to me. For one thing, it means two code paths in pretty much every procedure dealing with a constrained resource (e.g. mempool, CNB). This makes code harder two write, harder to maintain, and slower to execute. As a result, some people have proposed eliminating BDD priority altogether. I have another idea.

We can create and maintain a conversion rate between BDD and fees to create a composite priority metric. Then we just do compPrio = BDD * conversionRate + txFee.

How do we calculate conversionRate? We want the following equation to be true:

sum(fees) = sum(BDD) * conversionRate * BDDweight

So we sum up the mempool fees, and we sum up the mempool BDD. We get a policy statement from the command line for a relative weight of BDD vs fees (default 0.05), and then conversionRate = (summedFees / summedBDD) * BDDWeight.

As an optimization, rather than scanning over the whole mempool to calculate this, we can just store the sum and add or subtract from it each time a tx enters or leaves the mempool. In order to minimize drift (the BDD for a transaction changes over time), we recalculate the whole thing each time a new block is found.