Head supershape i-magnum advice

That chart was dreamt up by a retailer, Head and most other manufactures don't publish sizing charts. There isn't any magic formula for choosing the right length for skis, the optimal length depends on multiple factors.

The general rule for piste and all-mountain skis is to pick a length somewhere between your chin and the top of your head. The ski lengths given by the manufactures refer to the length of the running surface and not to the height of the tip with the tail stood the ground.

Put your boots on, measure to middle of nose and conclude that 170 is the correct ski.