I have flushed while fasting. Twice in fact - on day ten and day 24.
If you go to the Liver Flush Forum , find a post by me and click on it and then go to the top of that post and click on 'show all messages by tiratu', you will be away.
When I was thinking about doing it, I read all the archives and everything I could find and I only found two others who had done it and they did not say anything about their experiences.
What I have read on this site in general is that it is very rough on the system mid fast and the recommendation is to do it at the end of a fast.
The first flush for me was like a walk in the park - so easy peasy. However, if you look at the Liver Flush Forum , not all first flushes are like that.And your experience of flushing will never be the same twice.
My second flush hit me really quite hard and looking back I didn't allow for how hard it would or had hit. I did it and sort of forgot that I had just done it.
I did find that after each of the flushes, my tongue was a lot pinker.
The way I decided about the first flush for myself was this. At day 10 I felt great - really strong, healthy and happy and a flush felt right. I had never flushed before and this was also my first fast. So this is not the voice of experience here - just an experience.
I got out tonnes of stones with that first flush and loads of crap with the second one. The second one just made me feel weak - I think would be the best was of describing it. And although my tongue looked alot pinker - I felt physically knocked.
All I can suggest is read, read, read and tune into what feels appropriate for you.