@Bedrockgames, have you looked at the dice schema you're using? 75% success rate is a 10 on 3d10k1. 50% success rate goes from a 5 on 1d10 to an 8 on 2d10k1. You might not have much granularity here. Especially if you're even considering allowing up to 3 extra dice (even if rare, I assume 1 extra isn't that rare, and that moves the needle significantly).
Granularity isn't a particular aim here. This is more a game for simplicity and speed of play. I find a rank of 0 ranks to 3 ranks works pretty well for what I am after and keeps the dice pools down to a reasonable level (presently the game has a soft cap of 6d10, and hard cap of 10d10). This is system I have used for ages, so I am not building the system itself (this is just a new iteration of the core system). What I am trying to do is fine tune how I talk about the different challenge levels so my language and numbers match peoples expectations as best as possible.
I have a number of probability charts I use working on this. The current one I use gives the probabilities for 2d10 take the lowest to 10d10 (all taking the single highest) for TNs up to 10 but also including probabilities for multiple "Total Successes" (10 results). The numbers are rounded up (which is an important consideration as that has some 100%s that are not actual 100% chances---I have other charts that handle the rounding in different ways). But overall this is the one I like to use (a player put this one together for me).
The best a character can have in a skill is 3d10. Even leveling won't take that higher. But they can get bonuses for situations and tactics, and some path abilities will also confer bonuses in some cases. But those bonuses will never exceed 6d10 (only monsters are going to roll up to 10d10 on things).
Presently I have been leaning on TN 4 for easy, TN 7 for challenging and TN 9 for Hard (I used to say 8 for hard but I think 9 is closer to what I mean by hard). The default TN is a 6 (and I have to admit I get a little OCD about that being one number away from a Challenging TN of 7, but I haven't been able to talk myself into lowering it or bringing it to the default). But if someone can make a good argument for TN 6 being challenging that would certainly streamline my TN advice.
EDIT: Also just to put those numbers to some probabilities clearly:
Easy: 4 (unskilled 49%, low ranked 70%, Medium ranked 91%, High ranked 97.30%)
Challenging: 7 (unskilled 16%, low ranked 40%, medium ranked 64%, high ranked 78.4%)
Hard: 9 (unskilled 4%, low ranked 20%, medium ranked 36%, high ranked 48.80%)