I'm trying to determine conversion for US. - dry teaspoons to ounces and grams, namely 1 tsp, 1-1/2 tsp, and 2 tsp. Every conversion table I've seen shows different values. The one I often found was: 3t, 1/2 oz, 14.3 g. If there is consensus, I'll just work it backward, but I really want this to be right!
Part of your problem, as stated, is attempting to convert volume measurements to mass measurements. You cannot do that without knowing the bulk density of the materials and tossing that factor into the equation ie. a tablespoom of cornstarch (UK corn flour) does not have the same mass as a tablespoon of fine salt. (In the USA, think weight instead of mass though it is technically incorrect) One great little program I use for conversions is found at https://joshmadison.com/convert-for-windows/. It is free! If working with a Roolander's recipe, try to remember that their tablespoons have 20ml volume not 15ml as is common in much of the rest of the world.