BTI treatment as prevention..?

I was wondering if anyone has used BTI to treat or prevent fungus gnats? If so can you give any idea of concentration? I bought dunks thinking they'd be easier to steep for a short time and reuse, but the directions are for large bodies of water rather than a small watering can. I read that it's good practice to make a stronger solution and dilute it to use as and when (within 2 weeks) but I can't judge how much to start with.

I hate the little blighters and see them regularly out of the enclosure, I imagine they're plotting so I want to make it inhospitable for them without ruining my perfect little microclimate.

Thanks!