As microblocks are rewarded 40% / 60% (https://github.com/blockstack/stacks-blockchain/blob/master/sip/sip-005-blocks-and-transactions.md#block-processing) it would make sense to use the received block rewards.
Instead of (Total number of testnet STX Blocks Mined) / (Total amount of testnet BTC Burned) use
(Total number of block rewards received) / (Total amount of testnet BTC Burned)
For the prizes, maybe use : Total prize money * (number of block rewards received of participant) / (Total number of block rewards received by all eligible participants)