More in general, if a level 6 challenge makes you do a DC 22 save or check, you would now no longer include the six points from level "built into" that number 22.
So you'd instead use 22-6=16.
You will of course realize this makes DCs by level nearly static. Saving against a monster's spells. Hitting their AC. Whatever.
Before the DCs for levels 1, 5, 10, 15 and 20 were 15, 20, 27, 34 and 40.
Now the DCs for levels 1, 5, 10, 15 and 20 become 15, 15, 17, 19 and 20.
Only the few points that mirror increased proficiency rank remain. You don't get a strange or dodgy system. You get basically 5E, which works just great.
(Don't get stuck on the fact that Expert, Master and Legendary DCs are now 5, 7, and 10 steps higher than the "corresponding" level DCs.

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