4:1 seems to be a sweet spot. People with capped Philo will be able to convert and probably be able to buy a single i90 peice with 500 myth. If SE wanted to be jerks about it, and I wouldn't put it past them, 5:1 will put someone without myth tokens just short of a single peice (as I don't think there's any <400 peices)
10:1 if they wanted to be total A-bags about it, and likely piss people off for a few weeks before they got back to grinding points for more months on end. The housing thing pretty much proved to them that they can swindle the player base pretty hardcore and get a clean getaway in the long run.