There is one small problem with your analysis.
There are actually two points that satisfy all the conditions you specify. They are on either side of the line between A and B.
(Specifically the defined triangle can be flipped along the A-B line.)
It's usually enough to look at the two locations and realize that solution #2 is in an aggro pile and look at the other one. :D
If you find a third point you can get a unique solution. I tend to just target point B, estimate where I'm going to end up (based on my eyeballs on the map, then run until the distance to point B is the right number, then target point A, and iteratively adjust my distances until I'm there, but that's just because I'm too lazy to do the maths.
That's pretty much what I do. I use two flags, take a couple unflagged measurements to check which side of the line is the correct one, and check my distances from those flags once I figure out which side of the line I need to be on.
BTW, Ranger portals are usable even when a Kitin is trying to eat you; taking a few hundred HP worth of damage will not abort your travel the way it would with a normal teleport pact.
---Do not assume that you speak for all just because you are the loudest voice; there are many who disagree that simply have no desire to waste words on you.