I'm not an experienced builder...I'm much more of an abnormal hobbyist.

That said...the most I ever tried was +6, and it helped that application quite a bit where I needed it.

It wasn't a race type of application though, it was actually on a stock cam 1972 400 daily driver.
I was really happy with the change and decided to try it around 75,000 miles when replacing the plastic timing gears.