How do I set humanoidhipheight exactly right as my character grows?

So I have a character, which only has 3 parts in it: head, humanoidrootpart, and torso. The characters grows bigger as time goes on, and the humanoid hip height must be readjusted because of this. The problem is that I cannot figure out a way to do this. I tried to use an equation by increasing the hip height with the size, but that makes the hip height go either too low or too high. I have heard about using the distance between the foot and humanoidrootpart as the hipheight, but I have no foot on my model. How can I solve this problem?

If you make the HumanoidRootPart scale with the torso and head then you can probably use the formula Roblox uses for R15 rigs like so: Height = (0.5 * HumanoidRootPart.Size.Y) + HipHeight. Not sure if you tried that but how frequent do you increase the size of your models and by how much?