I would dismiss MoreBeer's calculator, except for arguing a claim with them.
As mentioned before,
Homebrew Dad's yeast calculator estimates haven't steered me wrong either. I have made well over a 100 starters over the years, many from 2-4 months old yeast packs, as that seems to be common availability.
Even the fresh vial from WhiteLabs' Yeast Vault I received a bit over a month ago took a good week to yield a decent starter, followed by a 2nd and 3rd one to ramp up cell count for 2 separate pitches. This was shipped under ideal conditions, no hot or frozen zones along the route.
This makes me wonder how much experience you have in making starters. Of course you should
combine both packages of low cell count and make a one starter from that. That's a factor 2 gain right there, which multiplies along...
Especially when you expect the yeast to be in poor condition, due to age, mishandling, stressed from overheating/freezing, etc. you'd actually start with a much smaller starter volume (say 0.25-1 liter) at a reduced gravity of 1.020 or even 1.010, to prevent any more stress on her weak cells. Then step up from there.
Yes, it will likely take 3 steps that way, and probably 2-3 weeks in total. But you'll end up with a good pitch of viable yeast. Save some out for a next batch, after putting all that work into it...
This is not to say you should accept MoreBeer's poor supply and handling of the case without friction.
Back in March I found 2 different WLP PurePitch sleeves of lager yeast in a 'yeast box' in the back of my fridge I never got to use. They were well over 2 years old! They both were revived with a 3 tier step-up, and yielded a large amount of yeast ready to pitch. The starter made with the older of the 2 actually lifted off several days before the younger one, she seemed to be of better overall health. This goes to prove that age doesn't mean everything.
When you often need larger pitches, as in this case, consider getting a 2nd starter setup. Remember: Factor 2...
You only need to buy 1 sleeve (really!) => make a 2 liter starter => as long you get good growth (good healthy yeast), cold crash and decant => split into 2 starters, stir concurrently => you should have your 600-some billion cells.
If growth in the first starter is less spectacular => cold crash and decant => make a new 2 liter starter before you split.