10 ppm would be easy to figure. We add SO2 all the time to our ciders and wines, at the winery. I run the lab and do free and total SO2 pretty much all the time (I AM the lab guy)
for each5 gallon, if you use 0.33g KMBS (potassium metabisulfite) . sssshhhh here is the link for easy calculations:
http://digimodern.com/labcalc/ We made it up from our SCO Unix program we used all the time. Now it's an online calculator. Try to keep it to yourselves.
I would think 20 ppm would not be discernable. The higher the pH the more KMBS you need to add and 20 is nothing at the pH of most beers. The calculator in the link also shows Molecular ppm (Free) for bottling, usually done at 0.9ppm Molecular. Just change the pH and get the amount needed. Remember, this is for wines and ciders, with pH ranges in the 2.98 to 3.70 ranges, even though it will figure higher and lower.