Sorry, Hotline...but your math is in error.
The heaters are not "constant amp load" heaters.
Here is how you calculate the expected load:
Assuming the heater nameplate says 240 volts, 1500 Watts. Using Ohm's Law, we find how many Ohms of resistance the heaters offer.
Now we look at the new voltage. Again using Ohm's law, we use the Ohms value, and divide 208 by that number. This will give us the amps at that voltage. Using that new amp number, we find our watts of demand.
Now, I'm just using "ballpark" math, but it looks like the heaters will put out less than 1200 watts at 208, which would allow you to comply with the 80% rule.
