The biggest king I caught last year (50) was on a 16lb 9ft tapered leader. I always use tapered leaders for fly turnover. That was an error, I thought it was a 20lb leader, it wasnt until I tagged and released it that I saw the empty leader packet and saw it was 16lb. Glad I didnt realise that during the fight
I sometimes use a 25lb bite or shock tippet about 400mm long. Only because the larger 30lb plus kings have hard mouths and long fights can sometimes mean they will wear through it. The 16b leader mentioned above was well worn and wouldn't have gone another 10mins. Fight time was just under an hour.
One note: I dont fish near rubbish. All of my fishing is sight fishing in water less than 1m deep over clean ground and I have to be able to see the fish so I can read it. Kings will suck in a fly without the angler even noticing and spit it out just as quick. Most of my hookups are in the corner of the mouth as the kings turn down on the fly (common feed technique) so the leader isnt anywhere near the abrasive parts.
A heavy tippet or leader wont help you squat if you fish over reefs. You'll likely just right off fly lines or break them. Once they rub their head on the ground in the first run you're over the worst.
Shy fish is normally a presentation error. Most of the videos etc Ive seen online are more of a display or "how not to" approach kings. When I hear people say "the kings were not on the bite (or chew)" particularly with kings on flats, that is 9/10 a user error NOT the fish. Kings dont go into shallow water to sunbath, they go there to eat. If a fish is swimming it is hunting. If its not swimming it is hiding, normally in a channel or marker. Not my cup.
I approach every fish as if it is a brown trout. Period.
Hope your 2018 brings you more light on these apex flats targets for fly fishing anglers.