Well, let's see. You want 16V at 1.2A from the regulator. But there are tolerances, so unless you use a trimpot on the regulator it could be as low as ~17V or as low as ~15.1. To figure out dropout, use the high number, 17V.
If it's a 317/337, allow for dropout voltage of 2.25V across the regulator, minimum. So the reg wants to see a minimum of 19.25V at its input. Allow a fudge factor of 0.2V (I'll talk about that later), so it really wants to see 19.45V at its input.
Consider that line voltage will be +/-10%. Your 20-0-20V tranny could be 18-0-18 on a hot summer day. That will have a peak voltage of about 23V, allowing 2.5V for diode bridge drops. With a 3300uF cap the DC voltage will be about 21.4V and the ripple will be about 0.87V. So your dropping resistor will be about (21.4 - 19.45)V / 1.2A = ~1.65 ohms. Use 1.6. With 1.2A passing through it, you'll dissipate about 1.2^2 * 1.6 = 2.3W, so use at least a 3W resistor (and 5W would be safer).
If you hang another 3300uF on the regulator side of that dropping resistor, you'll drop ripple to about 0.22V, close enough to your fudge factor to be okay. Bingo, you're set for worst case high regulator voltage and low wall voltage.
What about the reverse? If the line voltage is 10% high, without going through all the arithmetic the voltage at the regulator input will be 26.7V and the worst-case low regulator voltage will be about 15.1V, so you're dropping 26.7 - 15.1 = 11.6V across the regulator, at 1.2A, which is about 14W. That'll take a good-sized heatsink but it's doable with a 337. I'd use a TO3 case myself.
That's a bit of overkill, maybe, since it's very unlikely the regulators will really be that far from spec (and you can make them exactly on-voltage with a trimpot), but it'll give you a pretty bulletproof supply.
Peace,
Paul