I've got a similar Clarke welder, but different enough that my settings are probably of no use to you!
I generally find the recommended power settings for different thickness's in the manual are about right (don't blindly use them though - use them as a starting point, and go up or down until you get decent penetration).
The hard bit for me was finding the right wire speed for each power setting.
The way I did it was to run a bead on some scrap plate while my son gradually increased/decreased the wire speed until it felt 'right'. There's a good video somewhere on the tinterweb demonstrating it, but I can't remember where at the moment...
I did this for every power setting and drew up a table which I have pinned on my garage wall. Again, I only use that as a starting point, but it's usually close enough that I only need to make minor adjustments as I go.
Gas-wise I started off with pure CO2, and was amazed how much better my welds turned out when I switched to an Argon/CO2 mix.
The guy I got the gas from said my welds would be 'warmer' with the Argon mix. I wasn't sure what that meant at the time, but to be honest I can't think of a better way to describe it!
I'm currently using 15%, and would go for 20% given the choice between that and 5%.
I'd still be happy to use pure CO2, but only if I could get cheap pub or fire extinguisher stuff like some guys on here seem to manage
Don't worry about asking lots of questions, that's what the forum's all about!
(P.S. I'm still a bit of a novice when it comes to welding, so take all the above with a pinch of salt!)