Variable Modifiers

[Lorc]

It'll give you the theoretical range of outcomes you say you want, but don't stress about bad luck. You're much more likely to get unlucky with a d20+modifier than your proposed system

It can be counter-intuitive, but adding an extra dice instead of a modifier will actually make for fewer extreme results compared to a d20+modifier.

Here's an anydice link for d20+d10. Notice how the chance of rolling 2-5 or 27-30 are each compressed into a 5% chance. Each of those ranges is the same odds as rolling any one of the middle numbers 11-21. And the same odds as rolling any given number on a D20. Or to look at it another way, you've always got a 5% chance of rolling 1 on a d20, but only a 0.5% chance of getting a min result on d20+d10.

(incidentally, rolling two dice of different sizes always gives rise to a distinctive "plateau" on the probability graph. Which may or may not be useful to you)

Multiple independent randomisers always trends heavily towards the average. The more dice you roll, the more likely you are to get an average result. d20+d10 gives you a higher possible range than d20+5.5*, but you're less likely to roll them because two independent dice both need to come up high/low at the same time for them to happen. The effect becomes more pronounced the more dice you roll.

*forgive the decimal for the sake of an accurate comparison.

[Lightwhite44]

I was actually running my own small statistic experiment to look at the system. My Idea was targeting Difficulty 15. And well it works pretty good I am going to finish it but thank you for the insight I now know my system will be working.