I have to strip up solid wood flooring glued to concrete. I am going to replace it with floating wood (engineered) flooring. Is it necessary to completely remove any trace of the glue or is scraping it flat sufficient for underlayment and floating floor (gonna use Quietwalk and Schoens Quick Clic)? This so called 50 year wood that is down now was a very expensive mistake!

What you are floating over is not near as important as how level / smooth the floor is.

3/16" over 10' is what I have often understood as the industry standard. However, I have also heard 1/8" over 6' has also been cited. This is also easier to measure. Most of us have a 6' straight edge. I have never seen a 10' straight edge, so I wouldn't know how to evaluate this standard.

Basically make it smooth and flat. I wouldn't worry one bit about floating over old adhesive residue.

Would that have been a LL floor? And Barquios is right, the slab needs to be flat. I would razor scrape the adhesive off enough to make it relatively smooth, then overlay the entire slab with polyethylene. I do not trust the 2 in 1 or 3 in 1 underlayments to be adequate for wood floors. Laminates, maybe. Wood, no. Then lay your Quiet Walk over that.