Hot take, but frost lines from decades ago are such BS now. The coldest regions of the state barely freeze down a few inches anymore except during the craziest winters. Go however deep you can while keeping a good slope. Definitely at least 18” but otherwise you’ll be fine.

Side note though, it’s hard to keep those things from clogging. And they’re expensive. If you can possibly swing it, just dig a shallow swale/ditch.