I have, over time, come to the tentative conclusion that the amounts of yeast in standard packages are just fine for most beers, exception being big beers (higher than, say, 7.0 ABV).
I attended a yeast workshop put on by Chris White of White Labs (yeah, the head guy!). I asked about starters and such and he said he'd just pitch a pack of his liquid yeast in most beers, no starter. Well, suffice to say that I was a bit surprised as that's counter most conventional wisdom in homebrewing. I've done that and <shock> it actually didn't change the outcome for me.
A packet of dried yeast will have someone in the neighborhood of 50-60 billion cells. A package of White Labs yeast will start with 100 billion, but over time that number declines such that it's probably, midway through its best-by life, down near that.
As a practical matter, I'd ask how the beer tastes. If it's fine, then the yeast is doing its job. You're doing what most people who are really into this homebrewing thing do, i.e., experiment. The best idea is to brew two beers, one using dry yeast at recommended dosage, another with a starter.
Now, there generally is a recommendation NOT to do starters with dry yeast. And truth be told, even rehydrating dry yeast is seen by many as unnecessary--though
Lallemand recommends rehydration now on their site.
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Another exception to the 1-dose advice generally is for lagers fermented cool, i.e., around 50 degrees. I make starters for lagers using liquid yeasts, but I do it in an unconventional way. I make my starter and try to time pitching the yeast at about 15-17 hours after I began the starter. I also oxygenate the starter wort. The idea is that the yeast is REALLY going about 15 hours after beginning the starter, and I'll pitch that whole starter directly into the wort. No crashing, no decanting, just right in at about 70 degrees. That additional liter of starter wort drops the gravity about a point, not enough to be noticeable.
I then leave the wort at 70 degrees for about 6-8 hours, and then begin a gradual reduction in temperature to 50. This effectively gives the yeast another doubling prior to the final temp drop. Another benefit is that if there's any contamination in the wort, the yeast can outcompete it because rather than sitting there going through lag phase and allowing contamination to get a foothold, it's working. Typically I have bubbling out of the airlock in 4-6 hours doing it this way.
And just to screw with your head a bit more, I also do an accelerated lager schedule. When the yeast is about halfway done, I begin ramping the temp up 4 degrees every 12 hours until 66 degrees, where I allow it to finish. This not only speeds up fermentation, it acts as a diacetyl rest.
You may or may not do any of this (and if you don't have temp control you can't), but besides giving you ideas, it shows that one doesn't just have to blindly adhere to conventional wisdom.
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Brewing is more resilient than we sometimes give it credit for being. As long as you follow good sanitation practices, and are close on temps, you'll be good. Enjoy the experimenting and see if it produces a noticeable difference for you.