I think for this kind of distribution it would be fair to have it capped or something. Like maximal redeemable amount is at 100 XRB or 1 BTC or something like that. It doesn't make any sense in linking to BTC balance since the ICO projects will claim their stake and just dump it because they don't care.
An amount cap sound like the right compromise. It can be further restricted by looking at amounts from a specific historical date, though I'm not sure how fair that is.
I'm guessing the coin distribution needs to ensure a (somewhat) fair distribution as well as introduce new users to the technology.
A captcha controller faucet, similar to XRB sounds nice, but seeing as XRB already did that, we could just look at the distribution from when the faucet stopped.
Taking into account that this is a DAG Privacy coin, besides airdropping XRB holders (as a thank you for the fork) perhaps we can also airdrop privacy coin holders (Monero, Dash, Zcash)? Users that already value privacy seem like the most likely to accept Stone and be part of the community.
Overall, a 55% airdrop distribution sounds like a reasonable amount.