I think most here will tell you its a matter of trial and error. It really depends on the size of the needlefish you have made, the profile, length, type of wood used, many variables. Some have in the past suggested turning your body and then use rubber bands to hold your weights on the exterior of the plug and see how it performs in the water. Find what works and go with it.
I use a tail weight and depending on the size and style of the needle I want, anywhere between 1-3 slugs of 00 buckshot further forward.
As I said it depends on exactly what you are looking for your needlefish plug to do, be it float, sink slowly or sink quickly.
Good luck.