Help! I just bought a Focusrite compressor/preamp and would like to know how to set the compressor ratios, e.g. 2:1, 4:1, etc I keep hearing about, as there's nothing in the manual covering this. I'm new to using a compressor and always assumed that these ratios were all set button/ knobs on a compressor unit.

Also, is there any rule of thumb as to what level to set the threshold at?