Is my math correct?
 

If your car at idle uses .25gph and you engine on coast for a mile downhill at a constant 60 mph, you have then used only .00416 gallons of fuel. That equates to doing this 240 times 'engine off' to save a gallon, or eternal vigilance!

60MPH/.25GPH = 240 MPG engine-on coasting @ 60 MPH.

If you can maintain the 60 MPH coast speed while in gear using DFCO then 0.00 gallons consumed, same as engine off coasting.:D

my car uses much more than that unless it's warmed up. my big EOC gains are when the car's not warmed up yet. Much less penalty for idling when warm.

That would be a steep hill!

It's better to think of it in a real world drive. If you do a 10 mile drive averaging 40 mpg for 9 of those 10 miles, and incorporate that 1 mile downhill, it looks like this:

EOC
9 miles @ 40 mpg.
.025 gal/mile times 9 = .225 gal consumed for 9 miles
.225 gallons consumed for 10 miles = 44.44 mpg
NICE-on
9 miles @ 40 mpg.
.025 gal/mile times 9 = .225 gal consumed for 9 miles
.25 GPH consumed for 1/60 of an hour = .25/60 = .004167 gal.
.229167 gallons consumed for 10 miles = 43.64 mpg

At a more realistic 30 mph, the NICE-on idle usage doubles because the glide takes twice as long, and the car uses .23333 gallons for 10 miles = 42.86