Replying to Mount point on 4frnt devastator

I am going to be getting a pair of the 179 dev's and am wondering where I should mount them. I will be skiing about 20% park 40% all mountain and 60% pow. for reference I am 5'9" and weigh about 130. the recommended mount point is 4.75 back from center so I'm thinking 3.75 back from center but I don't want my tips to dive in pow.

Also, should I get the 179's or the 172's? I'm leaning towards the 179's cause I'm still growing.