As long as you have reconciled the fact that it is just a guess, then yes. If you have a decent estimate of your starting count, say by buying a new yeast vial or smack pack, then making a starter from that, you can use yeastcalc.com to come up with an estimate of how much you will have after the starter has completed.
As an example, say you make a 1L starter from a fresh vial. According to yeastcalc, based on the viability date on the vial and the fact that you assume that you're starting with 100B cells, let's say that you should have 200B cells when the starter completes. Now, maybe you need 160B to pitch in your beer and you want to save the rest for a future starter. So, you're pitching 80% and keeping 20%.
A 1L starter + the 100ml or so of yeast and liquid in a vial of yeast means that you should have a total volume in your starter of about 1100ml. 20% of 1100 = 220ml. It just so happens that an 8 oz = 218ml, so you swirl up the starter to get all the yeast into suspension and fill up a sanitized 8 oz mason jar with liquid. It's a reasonable guess at this point to assume that your jar has about 40B cells in it and there are about 160B remaining in the starter flask... give or take. Label the jar with the quantity and date the starter finished and save it for next time.
After a day or two in the fridge, that 40B cells in the jar will settle and you'll be able to see what about 40B looks like, volumewise, in an 8 oz mason jar. After doing this a few times, you'll be able to take fairly educated guesses on how much a particular amount of yeast represents in billions of cells. Many times I've looked at a jar and thought to myself: "there's about 1/2" of yeast, so I'm guessing that's about 100B cells right there", then used that number as a starting point for yeastcalc. Is it really? Probably not, but it's close enough to get the job done.
That's what I do and it's served me well, as far as I can tell. I don't worry about being off, cuz if I did I'd drive myself insane.