This degree of precision is really a bit of a moot point anyway, since we are using volume measures to approximate the mass of solids!
The actual mass, in a given volume, can vary with all manner of things (packing density, particle size, moisture content, etc) and, for spices, the resultant intensity of flavour varies more significantly with their freshness!
We need to get real (and practical) about the degree of precision required. It is obviously far more significant to be clear if we are talking about a level or a heaped spoon that is being specified in a recipe. I reckon specifying any further degree of precision is pointless.
I use 1 level tsp = 15ml, 1 level dsp = 10ml and 1 level tbsp = 15ml
As far as I'm aware this is the case in NZ, Canada, USA and UK
Ray is correct, a tbsp in Oz is 20ml
If you want to get precise about solids, use grams!
Personally, I'm far more concerned about being clear about what a recipe means when it refers to "coconut powder", "tomato puree", etc :
