It might help if you described what your machine looked like.
I’ve never seen a snacks vending machine that sold any kinds of nuts or loose munchies without them being in a package, which kind of negates the premise of the question… you put in the appropriate amount of money, and you get whichever package is foremost in the selected slot. (Also, around here they normally don’t have knobs as much as buttons.)
There are old style vending machines which sell nuts and some other items similar to the way M&Ms, Skittles or maybe Chiclets are sometimes sold. That must be what the OP is referencing.
As I understand it, it doesn’t matter. The first part of the turn opens a hole that fills a cup. The cup will get totally full no matter what. The last part of the turn empties the cup into the chute with the flap on it.
I would suggest some gentle shaking or vibrating of the machine prior to your purchase, to help the product settle in to the portioning cup. Your aim should be to settle as many pistachios as possible into the cup, eliminating empty spaces.
This is my suggestion as well. Slightly off topic, always check the chute before you turn the knob. Some pranksters will put the ones that fall on the floor back in the chute. The next person turns the knob and gets a handful of clean nuts, plus a couple of dirty, germy nuts. Ewww.
Half a turn is usually right. You want the cup open and under the pile of pistachios. Pistachios don’t flow very evenly, so a little shake will ensure that they fall into the cup where you want them.
I’ve had some luck with certain types of machines. One I came across recently had a ‘feature’ where you could shake the handle from stop to stop before putting the quarter in and sour tarts poured out. They were terrible but, hey, Free!
Another type you can check for: The handle doesn’t have a spring return. This might be hard to explain…this type of machine’s handle doesn’t have a springy feel to it. THis type does not return to horizontial if you give the handle a little turn prior to feeding a quarter. These aren’t common but M&M’s might as well be free.
In general, I’ve had varied luck just shaking the handle. Place quarter, turn handle slowly, listen for items falling. As soon as you hear ‘action’, carefully turn handle from side to side but not so much that it clicks to the next index point. It may or may not work.
If the nuts are in shells, it doesn’t really matter. If they aren’t, what makes you presume that the chute is any cleaner than the floor? People touch the chute with their germy hands.
Buy pistachios in bulk from your local co-op. You will receive about 10 times as many pistachios for your money, and they don’t go bad. Keep them in a ziplock bag and you are good to go.
Third grade science teacher? Nuts! What we have here is a Doper in the making.
Here’s a better plan for you, Nature’s Call: Don’t buy that roll of quarters. Get yer heinie to the grocery store and buy the makings for a week of lunches and a bag of pistachio nuts. You only have to do it once. Between the $10 you don’t spend on the proposed experiment, and the money you’ll save on the week of lunches, you’ll have enough dough to buy a membership on the SDMB.