The included angle is the angle between the cutting surfaces on opposite sides of the tool. Countersink tools usually have the opposite sides either at 90 degrees to each other, or 60 degrees to each other. 90 is more common. So, it cuts surfaces that are each at 45 degrees (half of 90) to the longitudinal axis of the tool. That is probably a bit more than the angle of the sealing surface at the tip of the needle, which seals off the petrol flow when it rests against the seat. To give it a decent chance of sealing without needing a huge amount of force from the float, you want a narrow anular surface to be cut on the seat by the the countersink tool - as narrow as possible, but of constant width all the way around its perimeter. Of course it must be exactly aligned with the axis of the seat - a round needle can't seal an oval hole.
The kind of countersink tool you need for this job has about ten cutting edges evenly spread around it. That helps to keep it centered, which in turn helps keep it from chattering when you rotate it by hand.
