What is a good ratio for a bonus to Spell save DC and spell to hit bonus

I'm creating a homebrew item that is meant for spellcasters that is meant to increase the likelihood of their spells to take effect/hit. Let's say I give a spell save DC bonus of +2. What would be a fair to hit bonus? Would it just be +2 again? To me I feel like they're not the same value but maybe I'm just overthinking it. I would imagine that going from DC 13 to DC 14 isn't a big deal but going from let's say DC 19 to DC 20 would be a big deal. Just like how I feel like most value from to hit bonus comes earlier. Like id value a +4 to a +5 more valuable than a +9 to a +10.maybe I'm just going crazy or is there math behind this? Anyone who can clear this up or give any advice to my ramblings would be much appreciated 👍