Again, it is not total tip weight per se that turns over a fly but to first approximation tip gr/ft. Weight and linear density are somewhat mixed, but “mostly” separate casting factors. If it helps to visualize - as the last few feet of line turn over the rest of the line behind it is basically just moving forward with constant momentum and can’t exert a force on the front. The ”weight” behind the turnover at any given point is proportional to the gr/ft there. We are talking about line physics here, not “rules of thumb” - but I agree rules of thumb are generally easier to use.
Even if you look at bobs grain windows you will see they are much wider than the “sweet spots” summarized above. Example, my 7/8 MKS likes a 550-570 skagit head but has a labeled grain window that extends out to 800. I find that setup casts 15’ of t14 (210 gr) with relative ease, so already a bit out of bounds even for the sweet spot.
These things can easily get confused, weight and linear density, especially if you end up using the same(ish) length tips most of the time, but sometime do try the experiment of casting twice the length of a tip material that you find easy to cast. This will change the dynamics but only minimally the turnover capacity, and the heavier tip will have marginally LESS capacity due to its length gradually attenuating the energy.
Just as I commented before about skagit heads being overkill for turning over most tips that actually get used to fish, once you get to t14 and heavier (as the OP was wondering about) for example, you are NOT at all choosing it for turning over your fly but obviously because you want the depth due to the greater sink rate. OK, maybe it’s still and issue for some kind giant 10’ Muskie or striper fly. But in most cases NO, with a skagit head you are usually choosing the tip and length (at the heavy end) for depth, not the fly.
Another strategy is to maintain the TOTAL weight of the tip you use while varying the diameter - for a fixed gr/ft smaller diameter means faster sink rate. You can do this for example with Rio
replacement tips which are density compensated to have the SAME gr/ft for all sink rates (float, int and 3,6,8 ips) in a particular wt set. These are more expensive than simple t-stuff, and have the disadvantage that the maximum sink rate is about 8 ips, so approximately the same sink rate of t11. So to get faster sink rates you need to use t14 and up.
There are multiple strategies for getting multiple depths with sink tips, like
1. (Old school, simple and cheap) use different lengths of the SAME stuff, like t11. 6ft of t11 will turn over the same fly as 18 ft (actually a little easier) in spite of the fact that the latter weighs 3 times as much. Disadvantage: you are casting a different total weight and length for every depth ‘setting’.
1.5 same length different stuff.
2. (Aesthetic solution, Rio
replacement tips) use the same length and same total weight for every depth setting but change the sink rate. Advantage - exactly the same casting mechanics for every tip. Disadvantage - can only get 8ips max.
Etc, etc. - there are other strategies for this as well. MOW tips have the consistency of 2 but are just “wasting” part of the length - basically 1 with a built in extension of the head. But fine if you don’t want to go that deep. Plus lots of people would prefer the shortest, fastest sinking tip that gets the depth they want.
At the lighter end of the wt spectrum you do have to chose sufficient gr/ft to handle flies you want to cast, but once you have sufficient linear density then all the rest is about what depth. When you are in the large gr/ft domain of skagit heads most of the time for the wt of rods and flies you will use for the fish you are looking to catch it will be “overkill”. If you want to use sink tips with more slender lines like scandi heads and so on (lower gr/ft at the end) then you are more likely to have to worry about having enough to turn over the fly as well as the other factors mentioned above.
But like a sledgehammer, if smashing is the intention then in most cases you will not have to worry whether your hammer is too light to do the job. That’s a skagit head.