How to move points proportionally to keep edge flow good? So lets say we have 3 points A, B, and C.

I want to be able to move C, keeping the points between AB and BC proportional.

How would one go about doing this?

This would help a lot with keeping my edge flow good.
Consider 


scaling 3 vertices first which preserves proportion with the correct mode of scale (center point)
then translating by grabbing.