I expect your new Clarke pro 90 is similar to my old Clarke pro 100e.
If that's the case try and stick to smaller reels unless you save a packet. They feed nicer and don't tend to cut into themselves as the reel is pulled.
I use 95% argon 5% Co2, which is lovely for a smooth weld, and very controllable, but against most advice I prefer pure co2 for thin panel welding, I.e outer bodywork.