Blade of Mace Quantity
Personally, I conceptualize an average blade-of-mace size, and use that to determine if I should tear a bit of or not. The outcome will sometimes be a bit more macey, sometimes a bit less macey. For me, it's part of the charm of a curry, and also helps finding out personal preference of dosage for mace.

The same problem exists with "an inch of ginger", "a 2 inch piece of dalchini": There's thick and thin parts of ginger, smaller or wider parts of dalchini. I do the same there: Consider what the average part looks like (i.e. for ginger, the more bulbous part seems to be main part, with smaller knobs on the outside), and use that to take your measure.

Ideally they would specify everything in tea- or tablespoons, which would at least help for grindable spices, but in my experience, most indian recipes don't when it comes to mace, ginger and dalchini (and of course garlic: Big cloves, small cloves?). (Although for ginger, they sometimes specify "a thumb sized piece of ginger", which is quite workable).

I just accept that sometimes it will be a bit more gingery/macey/cinnamony, sometimes less. I have never had a failed curry because of it, it's just that the taste will float within a certain (acceptable) range of flavors.