Quote from dumb_mother:
well least amount of contracts moves you out further on the curve- the simplest balanced one would be a zb fly -2 ub, +6 zb, -5 zn
although someone posted earlier questioning the validity of the cme provided spread ratios- so maybe there is an easier one.
personally i wouldn't trade flys unless you can put atleast 3 units on, and with that one you'd have 18 zb and i've seen that fly move from top to bottom of an intraday move ~10k if you had all 3 units on, so keep that in mind- not always easy to take small losses on the zb fly because both zb and ub move in full tics, and ub is like a ghost when you need liquidity.