When adjusting the blades, either inward or outward, it is important to maintain full contact between the flat side of the wedge and the blade. The wedge is the washer-like piece that clamps the blade into the blade slot. Be careful not to over-tighten the screws.
It isn’t necessary for all the blades be adjusted to the same position relative to one another, but it is important that each blade extends completely over the surface it is cutting, no matter what the angle. That is, both ends of the blade should not be contacting metal.
When changing the blades make sure that the beveled end of the blade points toward the center of the cutter body. If the blades are installed backwards they will not cut and may dull prematurely.
Blade life will depend on many factors, including the hardness of the seat materials, amount of stock removal, turning method, etc. In most cases a set of blades will last for hundreds of seats. Neway cutting solution will extend the life of the blades as well as improving the surface finish of the seat.
Every cutter has its stock blades, but at times it may be useful to use an alternate blade, usually to extend the reach of the cutter. All of our carbide blades are the same height, so it's perfectly fine to swap them around, within reason. There are certain blade/cutter combinations that would not be a good idea but the wackiness of those combinations becomes immediately apparent when/if attempted, so we won't bother to describe them.
Here's an example that illustrates the pros and cons of blade swapping. It's not unusual for customers to order cutter 122 (1" diameter) with an alternate blade, TC250. The stock blade for cutter 122 is very short (5/16" long) and installing the TC250 blades, which are 3/8" long, extends the outer reach of the tool. The tradeoff is that the longer blade will extend out beyond the cutter body diameter, even when the blade is adjusted inward as far as it can go, and this protruding end of the blade may catch on a combustion chamber wall even though the cutter body clears the wall. This is exactly the reason that some 1" diameter cutters have the short TC253 blade (5/16" long) as their stock blade.