I'd say it's more exponential then linear but that is my opinion, and they do effect people differently so it's hard to say.

I use an expensive Mg scale with a .10Ml lab scoop to dig the stuff out of my jar and that scoop loosely filled level to the rim is around 150mg and the bowl part is about the size of an airsoft BB. So a "knife tip" is not a very accurate way to measure your doses, I collect knives and every tip is different. Best to get scale, you can get decent ones for around $20 on Ebay.

Might I ask what kind of knife, if it's a pocket knife...you've sparked my interest...