I found that I can't significantly and permanently change the way I kick to adjust to a fin that's too stiff or too cumbersome. I can do it for short periods of time as long as I'm making a conscious effort, but inevitably at some point I slip back into the most efficient kicking style that particular fin demands, which will be wrong as you described if the fin is too stiff. A prolonged effort has that effect with all equipment we use. The body finds a way to adjust in the most efficient manner regardless of how we think it should perform.
Past deciding that you'll be getting carbon blades, and choosing a company who's blades are known for durability and have a warranty for failure under normal use, stiffness and flex pattern are the most important factors in determining the right fin for you. It starts with the footpocket that fits you best and is most comfortable. Then a blade which combined with the characteristics of that particular footpocket will give you a fin with an overall stiffness and flex pattern that's perfect for you.
Normally the change in performance from plastic and even fiberglass fins to carbon is so significant, that the diver will perceive a big improvement as long as the fins are not extremely stiff or extremely soft. Most divers will then stay with those fins even if they're not "ideal" because the body will make small permanent adjustments without them noticing. The ones that don't are divers that have particular issues like knee or ankle pain, or divers that are very in tune with their performance and want the ultimate. Such divers now have a point of reference with their current carbon fins and can infer whether a softer or stiffer fin is needed. For me selecting the right stiffness is well worth the investment. To give you an idea of how far I go, at a time when the right stiffness footpockets were not available to me I weakened the tendons of my Pathos footpocket 3 notches at a time on the top and bottom of the tendon, testing the fins each time, until I achieved the stiffness and flex pattern I wanted.
For a long time I wanted to take the guesswork out of buying carbon blades. I've achieved this to a degree by familiarizing myself with the different footpockets and blades available on the market. Then if a diver can't bring in their fins I still more or less know what they have if I'm given the brand and model of the blades and footpockets. And can then deduce the fins they're looking for using the old fins as a point of reference. This doesn't always work because I can't know all of them. For example in your case I'm well familiar with the characteristics of the Nemo footpockets, they have a nice stiff sole and a little stiffer than moderate tendons. I can also say that the manufacturing has been consistent. But I have no idea about the C4 blades. I've seen some that were soft like noodles with a poor flex pattern and useless IMO.
For someone coming to my shop we'd simply go to the pool and have them try a few different stiffness blades. If I was dealing with someone I couldn't see in person, the best approach to get the stiffness and flex they're looking for would be to send me their complete fins and let me know in percentage how much softer the new ones should be. Not difficult to achieve if the same footpockets will be used for the new fins. A little more involved if they're looking for different footpockets.
I cut the tendons off H Dessault foot pockets and reshaped them. I figured the blade will need more support from the bottom so I shaped a piece of starboard. It doesn't feel much different than a blade in Pathos foot pockets. It will feel different once the water channeling rails are on. I'll try it and post back. Xan, you're welcome to try them.
