EDIT: This is for 2007scape.
Made an account for Sythe because I saw this thread.
I have a staking odds calculator. It is 100% accurate (literally, it uses the ingame combat formulas).
It is basically a simulation: You input your stats, opponent stats, type of combat and it duels a million times for you, and gives you the results of the stakes. It even shows you what you can X or be X'd to make a profit. (on average)
I am the only person who currently has this, and I shared it with no one else in the world. (This is also the reason it looks super basic, because I don't need a pretty UI just for myself)
I have a couple of simple presets which are dds -> tentacle, just tentacle, rune knives, dds -> dscim, etc. However, there's manual inputs you can use to simulate any armour and weapon combination in the game.
I started using this calculator 3 weeks ago, and have since then made 1.8billion gold (starting from 85m) and sold 1.5bill for 1590 dollars. I am only willing to sell one or two of these copies, and I will package the source code so it can't be resold, because the calculator will lose a lot of its value when it becomes more widely used.
Screenshot: