What I like to do is use the Damping Ratio shown in the chassis setup/shocks setting tab. First put in your 1.1x4 with 300, see what that damping ratio number is. Then choose the 1.6x2 pistons. Next, change your oil until you get the same Damping Ratio number. That should give you the same feel. (for low speed, or handling)

One thing to add... the pack will be different between the two pistons (most likely), so you will want to go to the Damping tab, and look at the Damping Ratio graph. Choose both pistons by ctrl-clicking each one to highlight them in blue. Choose your range of viscosities to try match what you had for each piston (since you can't put a separate viscosity for each piston) so the values land on your oils you found to match damping ratio from the first part. The low speed damping, (what you just adjusted using the above steps) should be the same at the 200mm/s area, because that is the region the Damping Ratio number is calculated. Now, to figure out the pack, look at the 2000mm/s area, and one piston/oil combo should be higher than the other, that's the one that has more pack.

You could also get the pack difference by looking on the Dynamic Response tab, and actually entering both piston/oil combinations as Current Setup, and Test Setup. Then, choose a medium jump for the graph (instead of a sine wave... sine wave should be overlaying if you matched them properly). The piston that dips lower, indicates it has less pack, as the graph shows the chassis height off the ground during jump landing.