"Critical point" of a mixture

[Fausto A. A. Barbuto]

Good day,

In a phase envelope similar to one of the curves shown in:

http://www.coolprop.org/fluid_properties/Mixtures.html#phase-envelope

and for a mixture containing 80% methane and 20% ethane, how would I pinpoint 

exactly (that is, within a couple of decimal places) the point where the dew and 

bubble point curves meet?

I don't necessarily have to have the phase envelope determined or plotted; any

other method (simpler or more straightforward) to determine this point would

do.

Thanks.