If it says 0 it will still send, but I highly recommend against doing this as you could mistakenly send to the whole database or the incorrect audience. The audience number is one of the last pieces of information you have to ensure you are doing things correctly.
There's really no "better" way to do this, but you could just run the email as a smart campaign rather than an email campaign. Then you don't have the 15 min wait period, but it will still take time for the number to calculate.
Other options including seeing what you can do about reducing the size of your database and to make sure you aren't using redundant filters and giving the system extra, unnecessary steps to go through.