I avoid that kind of method because it reallies on the fact that on several arbitrary tgins which you might lose track of after too much time not using it... like 0.0001% is 1ppm ... oh I am not sure now, might be 0.001 or 0.00001%...
So I prefer to just simply calculate it...
% = 10-2
ppm = 10-6
So just move the decimal point 4 positions one way or the other, to convert one to the other...
0.005% time 10,000 is 50ppm, take a split second and can't go wrong...
To speed it up one might remember it's 4 decimal places... but if in doubt, takes a split second to figure out a million divided by one hundred is 10,000....