...if worn they need to be comfortable and allow easy access.

Now that I have your attention, gentlemen....

A lot of how much hilt should show depends on how close to the knee cap you wear you hose. If the cuff comes all the way up to the bottom of your knee cap, then only the very top of the sgian will be visible, it won't be very comfortable, and it certainly won't be easily accessible.

Most agree that the kilt looks its best if the top of the hose cuff is about 3 finger-widths below the knee cap (this will vary somewhat between individuals based on stature and type of build, but generally speaking the "3 fingers rule" works best, at least as a starting point). Coincidentally, the hilt on most sgians dubh are about "4 finger-widths" in length.

The top of the sgian dubh should more or less line up with the bottom of your knee cap. This means that if you follow the "3 finger rule" for your hose, you should have about "3 fingers" of hilt showing on your sgian dubh, with the rest comfortably tucked down your hose and held in place by your garter.

Hope that answers your question!