Not enough yeast cells for 1.058OG?

Brewed my second batch of homebrew two nights ago -- a 5 gallon American Amber Ale extract kit. OG was 1.058. I put in one smack pack of Wyeast 1056 directly (no starter). Checked fermentation at 24 hours and there was significant krausen and blowoff activity. Felt that things were going really well.

Today, upon reading various threads in the Fermentation & Yeast forum, I am beginning to fear I did not provide enough yeast! Using the mrmalty calculator I found today http://www.mrmalty.com/calc/calc.html it seems that I should have used two packs of 1056. mrmalty calls for 200 billion cells whereas my single Wyeast 1056 pack has 100 billion.

Question: did I under pitch? Should I pour in one more pack tonight, 48 hours after original yeast pitching, or is it too late to do such a thing?

Thank you for your feedback!

More info, I am fermenting at 68 degrees. Wyeast pack is 1 month after production.

Yes, you probably did underpitch a bit, but it's not enough to get worried about.

No, there's absolutely no reason to pitch another pack now. The yeast have already grown way more than 200 billion cells by this point.

You likely did under pitch a tad but with the smack pack being as fresh as it is you will be just fine. In my humble opinion the MrMalty calculator over estimates that amount of yeast needed. My last batch I was pitching a washed US-05 slurry that was a month old. Based on the age and adjusting it for how clean and compact the slurry was the calculator told me I needed 40% more than I had to pitch. Not wanting to wait a day or two for a starter I went ahead and brewed my batch and pitched what I had in my mason jar. I had violent fermentation in less than 4 hours at 58 degrees. It was by far the most violent fermentation I have ever seen from this yeast outside of pitching directly on a yeast cake.