For boxes, 1.125 silvers per box (combining 40% and 90% halves - about 20% of my box finds are 90%). I don't do nearly the numbers other folks on here do, and I've never found anything close to a dream box, but I suspect something in the area of a one-per-box average will hold regardless of what I do.
I've never found CWRs of halves (other than some obvious dumps), but have searched MWRs and loose halves. My records of loose halves searched are not perfectly kept, but loose halves and MWRs (outside of boxes) have produced minimal silver for me. I still think searching outside of boxes should produce better averages, but since there appear to be no CWR halves in my area and rare loose halves, I rely on boxes to produce silver.