This isn’t the way you should approach the problem. Don’t look for a dev who knows astrology. Look for a good dev, and write a good design document. If you explain clearly in writing and wireframes what you want, your coder doesn’t need to know about astrology. You’re the astrology expert, so they can follow your directions. All they need to know is how to code. I have a business recruiting developers (including freelancers), and when I search on behalf of a client, I always focus on the skills more than the knowledge domain.
Up front. Never hide compensation info. If you do, you might waste your time screening a candidate, only for them to withdraw when they discover that they don’t like your terms.