Creating an input field that responds to Sums?

[Aetheraeons]

Hello! First time trying something out like this, very inexperienced :(.
First, context (unfortunately neccessary): The goal is to create a field / set of fields to represent an TTRPG character's Current and Maximum Health. I want there to be a dynamic way to change it in frontmatter.

• So far, I have a table that configures the maximum health (image 1).
  Max-Mod adds a bonus / detriment to the maximum health; Temp HP functions separately.
  
  VIEW[{HP.Roll} + {Mod.Con} * {Level} + {HP.MaxMod}][math:HP.Max]

• The strategy at the moment to display Current Health looks like this:
  
  VIEW[{HP.Max} - {HP.Damaged}][math]

My desire is to create something similar (though nowhere near as complex) as D&DBeyond's Hit Point Tracker:

Effectively, I'm looking for a way to:

• Keep the Max HP field a formula
• Have Current HP as a field drawing from it as the base
• Have some way (potentially separate) to input a number (instance of damage/healing) and have that change the Current HP, with those numbers being able to be "stacked" (essentially we're looking at something being able to process a sum).

So far, I've been alternating between the manual method (fine with smaller numbers but requires counting and continuously remembering to add it all together), and a progress bar i made:

INPUT[progressBar(minValue(0), maxValue(42)):HP.Max]
Arguably the Progress bar works rather well, even though I do need to manually move it in response to Damage or Healing. Unfortunately, I would have to manually adjust it whenever the MaxHP changes, as that input doesn't allow anything but Numbers.

Any advice would be helped! If anyone's seen similar things made, I'd love a link to them.