I think I agree with your definition. 
I’be always pictured it something like- the line where you can make decisions differently. 
The poor have to look at immediate impact of decisions. Middle class can afford some luxuries, but more importantly can afford to look at decisions in the long term (building a nest egg, investing in their own home, buying a good car instead of just maintaining a crappy one). 

But I grew up poor, so maybe that’s just what t means to me.