Quite often it's necessary to fit a nut or washers underneath the top to get the height right, and ideally a star washer to stop the pot turning. With a washer and nut on the top, there should be a turn or so of screw thread showing. Sometimes pots come with two nuts, sometimes only one. Don't know why as the cost of an extra nut is negligible and they are often needed. Can't tell with the knob on, but the shaft to the right of it looks like it would benefit from sitting slightly lower, which an extra washer or two underneath might sort out.

If they are the knobs that came with the kit, then they should be fine as regards fitting the shaft properly. There are two types of splined shafts used for guitar pots. 18-spline and 24-spline. Asian-made pots typically use 18-splines as a default. USA-made pots like CTS come as 24-spline by default (though if you look hard, a lot of manufacturers can supply both spline types).

Knobs for 18-spline shafts don't fit 24-spline shafts and vice-versa. They will just sit on the top of the shaft, (pushing the two halves of the shaft together) and not fit properly. So if you buy aftermarket knobs, make sure you know your spline count! I have come across 'universal' knobs that don't have internal splines but a soft plastic ring inside which just grips whatever splines are there ,and they are quite good if you are unsure.

And even with the right spline count, I've found some knob designs don't push down as far as others, something you only find out from experience as to which ones work best. It's been a while since I've ordered any, and I've now forgotten which ones I like best. I should have written it down!