As long as you have the correct amounts the 8 parts paint:1 part activator:2 parts reducer, order added is somewhat irrelevant, as long as it all gets mixed in and in the correct amount. If your cup doesn't have an 8:2:1 scale, then use ounces marks.add 8 oz, then add another 1 oz, then add the last 2 oz.(8 oz mark for color, 9oz mark for activator, and 11 oz mark for reducer, or 16,18,22, etc) Or make marks on a mixing stick at a measured line (IE every 1/4" or 1/2" whatever works best for your application and amount needed), just be sure you have a flat bottom can with vertical sides (IE metal paint can) your mix will be off if using a tapered type cup. You could also use the 8:2:1 scale on the cup if it has that and just make sure you are adding the reducer at 2 parts, then the activator at 1 part. Several ways to do it and I am sure some others that I have over looked. Also, if you have a tech sheet for the paint make sure what products are used at what amount, usually will tell somewhere on the label as well (IE 8:1:2, the 8 is color, the 1 is likely the activator, and the 2 is likely reducer, but make sure that is correct for your particular paint.) Sorry if I made that a lot more complicated than it needed to be.
Kelly