White Labs Yeast Viability

WYeast packs show the production date and in the past I have used this to calculate viability when making a starter (assume losses of about 20% per month). First time using White Labs. Rather than a production date, White Labs has a “Best Before” date. Does anyone know how to estimate viability? Maybe the production date can be known from the Best Before date and the same 20% monthly loss can be assumed.
Thanks in advance for any guidance.

The production date for White Labs yeast is four months previous to the best by date on the package. I use the Brewers Friend pitch rate/starter calculator for viability estimation and making a starter.