Using a variable from one sheet in another

So I recently started playing as a Drakewarden and they have the Drake companion which has his hp calculated as 5 + five times your ranger level, I was wondering if I could auto-call the ranger's level to have the hp automatically update whenever I level up by calling that variable from the other character sheet 

Assuming you are referencing the 5e by Roll20 sheet the answer is: Not without a Mod (API script). The HP fields for NPCs and PCs are not auto-calculated.  The solution I use, without using an API script, is to simply make a note of the things to do to the pet (such as a Drake companion) when the PC gains a level. I put it in the Bio & Info tab of the companion or on the NPC sheet in the abilities section. 

Is there a chance you know of an API that does that?

I expect you could do something like that with either ChatSetAttr or ScriptCards.