CPM-S35VN makes excellent fillet knives. It has a good stainless resistance to corrosion, and has a sharp and tough edge. I would use .09/.10"" thick for larger and heavier fillet knives ( tuna, rockfish, drum, etc.), and .06/.07" for smaller fillet knives. Use less distal taper for stiffer blades ( less flex). On a long thin fillet blade, there is no real need for taper except for the last 1/3 to 1/2 of the blade to the tip. The thinness of the steel gives plenty enough flex. I like the tip ( measured about an inch from the point) to be half the thickness of the ricasso.
I make a large fillet knife with a 1" to 1.25" wide blade about 10-14" long. Small fillet knives are usually 3/4" to 1" wide and 6-10" long. For long blades with good flex desired I use the .060" steel. These type blades are popular with flounder and other flatfish fishermen.
Handle type is important, and varies with different type fishing and personal preferences. Some are pretty much the same as a kitchen knife, and others have a deep hand recess, like a fighter, to provide a strong and indexed grip. I like a good recess for the first finger and a slight palm swell with just a bit of a birds head butt..
Avoiding breakage on a fillet blade is usually done by having more flex, not less of it.
Hardness and steel type do not govern flex. Toughness and geometry (thinness and taper) does. Mostly, it is geometry.
The edge life is a product of the steel type (chosen for toughness), and the hardness from HT. Hardness and toughness greatly governs edge retention.
Toughness and hardness are a sliding scale where one increases as the other decreases. You pick the attributes you need/want and find the best balance.
Most makers find a tradeoff that gives the best of both. On a fillet blade in S35VN, I generally use lots of taper and HT to around Rc 61-62.
If your customer wants a long and stiff blade with long edge life, I would suggest you use the .090" CPM-S35VN stock with about a 50% taper starting at the center of the blade. Do the HT ( or have it done) at a point or two lower than you would for a kitchen blade, Rc61-62 would be good.
First, I would draw up a couple fillet knife silhouettes and let him pick the shape he wants. Draw them with 14" blades and shaped like a long boning knife. Let him re-draw on the sketches where he wants the tip to be and the tip shape he likes. Make other adjustments as per his wishes. Once you have a good sketch, proceed from there in steel.
If you have a piece of .060" and .090" stock around, show him both. Some people have no idea what the thickness numbers mean, but can see with their eyes what they like.
I have people ask for a kitchen or fillet knife and say they want it 1/8" thick. First, I show them a piece of .060" stock, and they say, "Yes, that's just right." Then I show them .125" stock, and they realize how thick that is.
