there are several things you can do.

  - First, like you indicated is: you can take less margin (and believe me, there's still considerable margin).
  - You can play with erasure coding policy. (n+10)/n makes that you can tolerate 10 failures. a higher n makes your storage overhead less. (which means more margin)
  - you can use even fancier storage schemes (fe online codes will make your storage overhead something like 3%)
  - you can use cheaper hardware.
  - ...