Sounds like a bug and if you can't get it down without crashing you are by definition over correcting. I would suggest don't both correct and also run + basal %. Do one or the other but not both.
What I would do is use corrections alone to figure out how much extra insulin you need at the moment to be stable.
Eg if you would ordinarily correct from 12 mmol to 6 mmol with 3 units over 4 hours but instead it takes you 5 units over 8 hours (take 3u, wait 4 hrs, test, BG high, take 2u, wait 4 hrs, test, BG ok) then you are needing an extra 2u/8 hrs = 0.25u/hr. So add that to your pump basal (just an example).
So basically use a series of corrections without a raised temp basal, then use the amount you had to over correct by, to raise your basal.
I would NOT use a % increase. Edit your basal profile to increase the dose, or better (if your pump allows this) copy your basal profile to a "Sick Day" profile and edit that copy.
Sorry that is probably not the clearest explanation. I have been grappling with this myself last 2 months due to having frequent infection.