George, two schools of thought:
1. Perhaps the answer is to remove the problem from the equation. My spoon is 18ml your is 15ml, so why not list in the recipe these quantities instead. So if I add 2 tablespoons of spice mix, I would write 36ml whereas you would write 30ml.
2. Does it really matter just so long as I keep the ratio of spices the same. Like when making a spice mix, if the ratio 2,1.5, 1, 1, 1 it doesn't matter what size spoon I use as long as I use the same spoon.