I had an entire response typed out with calculations for reverse-engineering our show rate...and then I remembered that it was worthless :)
To echo Alex and Erynn, there are two things you should be shooting for:
1 - a daily level of activity that feels better than being alone
2 - keeping an eye on OVERALL daily utilization to see if/when you need to start a waiting list on certain levels of membership.
For the first point, this is the hardest for most new coworking spaces, especially when they open without a community.
One thing you may want to do is evaluate your membership model and make sure that it hasn't dis-incentivized coming in. Punchcards, for instance, seem convenient but put the emphasis on days as transactions. It makes people think "Do I want to use my day today?" instead of "If I don't go in today, I won't get the value I paid for".
For the second point, like Erynn said, you'll be surprised at how wrong you can be about projecting attendance :) But, we've never been 100% full (on purpose), and here's how we did it.
We keep a daily attendance for approximating our headcount historically. But more importantly than that, we're on the lookout for days where Indy Hall is getting close to full.
We use waiting lists to slow down new memberships at the higher levels of utilization (full time and lite, for us). We have a finite number of full time spots, naturally, so that waiting list is pretty self explanatory. We clear that list first in, first out.
Sometimes, especially during our peak growth seasons, we'll also do a waiting list on lite membership, since it has the highest impact on availability. It might just be for a month or two, while activity stabilizes after growth. Once it's been a few weeks since we've had an "almost full" day, we'll open up new memberships again, first to the waiting list and then in general.
-Alex