[Amps] Emission numbers

[Ka9p at aol.com]

I've looked around the internet without success so I'll try here.
 
Can anyone explain what a good/bad emission number is on a used  
transmitting tube like an 8877?  Or just point me to an explanation?
 
I'm considering a few tested used ones, but I realize I don't know what to  
expect from knowing  a commercial testing result such as 80% emission where 
 the other numbers appear to be good.
 
Thanks, Scott ka9p