Making an educated guess of Cd.
 

It started out at .32, I have before and after coast down times. How do I calculate/guesstimate delta?

Good question, I think it has been answered before, but I want to see it as well.

Here's the Instructables page on doing a coastdown test:

Measure the drag coefficient of your car

this is not what I asked for. I have seen this before, but too much is assumed with this chart, things I do not know, or have any way to find out. I know the Cd of my car before mods. I just want to know delta (change), from there I can calculate the new Cd. this would be far more accurate than guessing all the variables that this chart assumes you know for certain.

for a rough idea, the % longer of coast down is ~= to $ better Cd.

Example: you coast 5% longer now = 5% better Cd or a change from .32 to .304 Cd

Actually, it is. You need to calculate your CRR before you can get anything useful out of coastdown data. A high-speed and a low-speed coastdown test, on the same road, on the same day, in both directions, is the way to go.

Or you could just look at the difference in coasting distance and decide whether the mod had a measurable impact or not.

Ehh.... I don't think that's accurate. Suppose I coast 50% farther. Does that mean I reduced my drag from 0.25 to 0.125? And if I coast 100% farther?

If you drive really fast so that aerodynamic drag dominates (see the calculator to determine exactly how fast), and hit the clutch, the rate at which you slow down (mph per second) should be fairly linear with CdA. But the mph disappear too quickly to accurately measure when you hit the clutch at 74mph.

True but it is just a "rough" idea for small changes. It is just another way of saying this mod helped or did not.