If you can't think of a good reason the community will accept NOT to make it 50/50, then you should probably make it 50/50.
(Otherwise, a competitor could out-bid you with a 50/50 clone.)
We could not think of a good reason to recommend less than 50/50 for BitShares X.
We have, however, suggested that a developer could justify reserving 10 or 20% to be auctioned AGS-style to raise funds to develop/support/promote a complex new DAC.
We have also suggested that some of the 80% could be used to attract customers once the DAC goes operational. For example a gaming DAC like BitShares bingo might need to have a supply of its own currency reserved to use as prize money until there are enough players to generate new prize money from revenues.
We suggest developers come up with a mix that will attract the most number of supporters and then ask themselves - how could a competitor clone a better mix?