The issue isn't that it's a crappy refrigerant, it's just that the two-temperature method is a crappy way to determine superheat with a blend.
If you use the temperature/pressure method based on the
dewpoint temperature, there's no need to be concerned about the glide. It's really no different than it is with a single-component refrigerant.
Where you need to be aware of glide is if you need to know what the evaporator temperature actually is...such as when you're initially setting up EPRs on a rack system. If the case specs call for a specific evap temp it's easy with a single component gas, but with a blend you need to determine the
mean evap temperature, which according to DuPont, is the temperature within about 65%-75% of the total glide. here's a link explaining it (See p.2):
DuPont Suva Temperature Glide PDF
Basically it's saying that for a refrigerant with a 10ºF glide, the mean evap temp will be approximately 7ºF above the bubble point...or 3ºF below the dewpoint temperature, so in JP's example it would be 18ºF-3ºF or 15ºF.