A fixed chance is good because it allows you to focus on abilities rather than the amount of hit points, and it saves players from unnecessary calculations, allowing them to pay attention to more interesting things and calculations.
I agree, a fixed chance looks more interesting due to its randomness. It's more fun and engaging.
I don't really like that when the HP ratio is 4 to 1, the chance becomes 75%, and when the HP ratio is 1 to 4, it becomes 5%. It's too predictable and mundane.
And how did you manage to get rid of the standard scheme? Is it very difficult? Or are there parameters that can be changed somewhere? Are they in the .xdb or .Exe file?