What target numbers would be a certain level of difficulty under this system?


I’m writing a homebrew game system, and I found that I have an action resolution mechanic but not a good system for target numbers (I call them Success Thresholds, or STs, in this game, and from now on I’ll use that term to refer to the minimum number a player gets that can succeed).

To resolve an action, most of the time players roll 2d6 and add a modifier ranging from +0 to +3, depending on the stat. With Advantage, it is (3d6 drop lowest)+mod, and Disadvantage is (3d6 drop highest)+mod.

There are also 4 (well, 5, but one auto succeeds) levels of difficulty. The Trivial tasks are automatically successful. Easy tasks should succeed about 75-80 percent of the time, Moderate tasks should be successful 50-60% of the time, Hard tasks should be successful between 25 and 40 percent of the time, and impossible tasks shouldn’t succeed more than 25% and often more like succeeding below 10-15% or the time even with Advantage and a +3 mod.

This is an anydice program with the base probabilities for a +0 mod. I want to know what number should the ST be for each level of difficulty? I had initially considered 7 as a base difficulty for Moderate tasks, before I added modifiers to rolls.