It's been shown through studies, using a peak-flow meter can determine whether you're in serious trouble or not and essentially "wait it out". The numbers vary from doctor to doctor, however though a seasoned asthmatic generally knows when it's time to either visit the hospital or use their inhaler.
That pretty much sums it up. They also cite the doctor's name so you can look more up on his studies.
Basically to put it bluntly, there are a million variables that come into play with something as delicate as asthma. That link below cites the doctor that conducted a study similar to what you're asking and also wrote some of the material.
This may not be considered an answer, but it's pretty close to what you're looking for. Most pulmonologists will tell you the same thing as well.
The "Should I tough it out, or should I get help" question is a huge debate.