Omega, If you have the option to get .010", .015" and the .018" patching I would start at with the thinnest and work your way up. IMO you don't want to use the thickest patch and end up being in the same situation Cayugad was in and have to hammer the ball down the bore to get it over the charge.
You want to be able to push the ball/patch down the bore with a fair amount of resistance (after using your short starter) and not have to bust a gut doing it.
In my patch and ball shooters I want a combo that I have to start with a smart smack of the short starter and a good then a good firm push with the 4" end of the starter. After that, if I had to guess, I would estimate that I was using about 15 - 20 lbs of pressure to smoothly push it the rest of the way down to the charge.