You could go -80 and push the red which is at 0 into the 80 and then pull that red or you could go 40 and push the green at 120 into the 80 and then pull the green to get the 80. Seems a little tricky at first but with a little practice you can pull any color with ease. There will be more coming I am just trying to keep it really simple at this point as to lay a foundation of understanding so I can correctly demystify this stuff.