Things get very vague at higher levels. Banksian Orbitals are rated at TL22 in WBH, but those aren't presented as anything like the most advanced examples of Culture technology. Niven Ringworlds are mentioned as existing, so that's about TL26-7 according to WBH. Likewise Dyson Spheres (TL28-9).
The Excession redesigns itself to be mostly engine and reaches a speed equivalent to 233,000 of lightspeed. One parsec per week is 169 times lightspeed, so that's 1378 parsecs per week, or Skip-14. TL33 if we extend the chart on HG p80. Given that's presented as an extreme case, Culture hyperspace travel seems to normally be in the Skip Drive range. Oh, and this is essentially using free energy drawn from the environment (though an antimatter universe may be involved). No fuel, antimatter or otherwise.
I'm comfortable with common Culture stuff being about TL22-25 (their equivalent of the Imperium's TL11-12, maybe), but I am seeing many examples of 25-30 in the books. And of course OUTSIDE and near the culture there are any amount of less advanced places, and some that can match it for a while. So maybe just go with double? TL24 is average, TL30 is high tech, TL18 is as primitive as you'd find within the culture?