Arbitrary number, since it appears quite apparent that there is no way to arrive at a "noncontroversial" number. To be frank, just used same number for GG and TBX.
Unless someone can propose a protocol for selecting a "good one", this number is as good as others
I propose that right now, a single individual owning 10% of the current coins "for the benefit of future development, bounties and whatnot" would be far less suspicious than someone owning 90% of the coins for those purposes. Everyone should decide what's trust threshold is.