You mentioned this is a mileage redemption? Why on earth would you pay such a high premium to sit in domestic F for the 1h hop to JFK and then again in regional business NRT-SIN? You're only spending 33% of your time in a premium cabin :| I'm sure they charged you 120K for the privilege too, but only if there was low space, if not what 220K? Did you consider JFK-NRT as your redemption for far less points and just paying cash for the connectors?

I would encourage you to reconsider... maybe check partner awards, especially KE Business? BOS-JFK on DL, JFK-ICN on KE82, ICN-SIN on KEnnn? I'm not sure what your travel dates are so I don't know what to suggest precisely.