I would taste the hickory smoke powder, yes, straight, and see how strong it is. Then taking a measured amount of rub, add small amounts and keep tasting until it tastes right. If you have it, use a 1/16 teaspoon measure and work up. You will get a sense of when it is right, assuming you have tasted your rub straight. In this manner, you will find the proper amount to use.
Although I like smoked salt, if you already have salt dialed in, you might want to use the hickory smoke powder instead.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."