I don't think you'll find any two answers that agree from anyone that's used cubeb. All the more difficult as few people who heard of it and even fewer who've used it. The reason being that taste is subjective. I love the flavour Szechuan peppercorns give food. It's often been described as being both hot and numbing. I can feel its numbing effect but it's not at all hot to me and I'm very sensitive to the heat from capsicum peppers.
Cubeb peppercorns are roughly the size as black peppercorns but instead of being solid, they're more hollow. I've only seen them in photos (otherwise I'd buy some to try) so I don't know if an equal volume of both would be an equal weight, roughly. If I were to substitute, I'd err on black pepper weighing a little more.
As for cubeb having a taste similar to black pepper and allspice, I couldn't say. But Gernot Katzer doesn't think so. Cubeb pepper (Piper cubeba L.,11
some sources seem to confuse cubeb pepper with allspice, which looks somewhat similar. In its flavour, cubeb pepper differs much from these other spices.
Other sites mention a camphorous note to cubeb. Rosemary is a herb with a camphorous flavour and scent. Cubeb is also supposed to be bitter. Many people don't like bitterness in food but I think it enhances many foods. What would chocolate be like without it?
What I would do, though not necessarily you, is to use around 9 black peppercorns, 1/8 teaspoon of allspice and a few needles from fresh rosemary, crushed and very finely chopped to approximate 12 cubeb peppercorns.
Hopefully, someone here who's used cubeb before will read your question and answer you before you've made your recipe.