Np problem, as long as the new writing on the Tippexed area is legible. Personally, I never use Tippex in an exam. The danger is that if I move on to other part of answer while waiting for it to dry, I may forget to come back to it.
My advice too is don't use it. Takes time and every second counts. A neat line through your error shouldn't impact legibility and it's only a draft you are intended to give - not a poliched article for dispatch.
