You get a completely different flavour profile using the powdered version so I'm not sure I'd even recommend doing that? Using the whole berries (they're dried as well) gives a very subtle flavour so if I had to use the powdered version I'd probably use very little, like a pinch or no more than a quarter of a teaspoon.

No supermarkets near me had anything but the powdered version so I ended up just getting my whole allspice from amazon so that solved that issue for me...