Damnation, I thought I posted to this, lol.
Anyway, the tranny you have is probably a 3.55 (check here:
http://www.neons.org/forumdodge/trans.htm to verify). This is kinda an opinionated situation.
I had the 3.55 in my daily NA eclipse for several thousand miles. The engine that was in there was a 10.5 comp engine, but other than that pretty close to stock. I didn't like the 3.55 because it was so "doggy". I ended up driving around in 4th most od the time. highway cruising was right around 2300-2400rpm @70mph. But it was too much of annoyance to me.
BUT some people liked it in their daily. So go figger.
Now, you might have the 3.94 with the .81 in there. That's what we have in our eclipse's.
As for the formula, I had ti at one time. Its on the internet somewhere, just google it up.
MB