Just wondering if I can just measure out 20mg by the tablespoon.
G, not mg...I assume you just mistyped it, but I wanted to point it out in case.
There's really not a good way to measure it accurately that way. From what I can find, if the SN is in finger grits, more can fit into the tablespoon. If the grits are bigger, then less fits. Also, many people don't understand the concept of a real tablespoon, and instead, they use an eating spoon to measure, which will throw off accuracy. Even the Gram to tablespoon amounts on this website may be off for this reason.
If you want true accuracy, a gram scale on amazon(USA for me) is a great idea. I got mine for $12 USD. It weighs in grams. It's really accurate, as it weighs a nickel to be 5grams(which is correct). Please consider buying the scale.
If you have a 4oz(113Gram) bottle of sn, I guess you could measure a 4th of it(~28Grams) and guesstimate it to 20-25. But the scale is DEFINITELY the way to go. I had spoken to
@jgm63 about this, and he is the one that believed the sn to have different sized grits.