At the moment beta sites like math don't have many tags, so I suggest moving the rep requirement to 101 (the amount of rep you get if you associate with another site).
It's fairly frustrating having to choose a tag unrelated to your question because there isn't one that is. I'm sure this puts off users who come from other StackExchange sites only to find that there are few tags, especially if you're on, say, an iPhone because the tag drop-down takes ages to appear, if ever.