I don't know that my heuristic is any good at all -- like you, I am unsure I'm playing BM well. But for what it's worth, here's how I approach it:
1. I look at the other cards. Is there a strong strategy? If so, I almost never buy BM.
2. But if the board is unexciting, I look at the BM deck. Does it feature cursers when the main board has none? Does it have a KC when the main board has a power card that can really benefit from one KC? Does it have a particular power card (probably NOT a trasher, since those lose so much ooomph if you buy them late) when the main board has none? Does it contain a major enabler that the main board doesn't? Particularly, if the main board has a non-village engine (like Lab or Hunting Party or Minion or Scrying Pool) and also several strong terminals, but no village, a village (or several) in the BM can be really good. Does the BM contain +buy when the main board gives you lots of coin but no +buy?
3. Preferably, does it have more than one of the things I just mentioned in #2?
4. Or finally, is the main board just so unexciting that I'm happy to buy BM as a terminal silver and just hey, maybe something good comes from it?
If #3 or #4, then I buy BM. Otherwise, skip it.
Related: It would be super awesome if there was a simpler way to view just the BM deck on isotropic. It would be slightly less super, but still awesome, if there was a simple way to view just the BM deck on isotropic already, and I'm just too stupid to know what it is.