I'm not sure what you mean by "dies down to keep alive"?? ..... do the rpms drop while you keep a constant pressure on the accelerator and the car is moving or is it more like the transmission is not engaged at first and the rpms jump until it grabs?
I don't know exactly what the numbers will be, they'll vary from one engine to the next even amongst the same type. Maybe there are some mechanics around that can give you specific experiences with the 3.8 but I haven't had to do mine yet and the last time I did this it was on a Chrysler 318 in the 70's. The key thing is that they're all pretty close to the same in each of the cylinders. Let's say for example that the peak compression is 400 psi in #1, 405 in #2, 403 in #3, 398 in #4, 410 in #5 and 400 in #6 then your car has "good compression". If #4 is 300 psi then that's an indicator of a problem, maybe a bad ring, a slightly bent valve or a head gasket leaking badly. The leakdown would be similar, say 5 of the 6 cylinders take 5 minutes to go from 400 to 300 psi and the 6th takes 1 minute. The one that loses compression in a minute has a problem, again maybe a head gasket, maybe a ring, maybe a valve seat.... Once you know there is a compression problem then you're pretty sure taking the heads off isn't a waste of time.