You can have that code automatically update the meter the same way as it does now by placing code like the following in a passage named "PassageDone".
<<updatemeter "health" `Math.clamp($health / 100, 0, 1)`>>
You'd want to have your meter set up something like this in your StoryInit special passage:
<<newmeter "health" 1>>
And in StoryCaption
You can adjust the size of your meter too if you want it thinner or thicker.
Explanation of why the code is more complicated.
This was a design choice. To make meters consistent, every single one represents a value from 0 (empty) to 1 (full). You can get that value quickly by dividing the value you want from the meter's maximum value, eg $health / $maxHealth. In your case that maximum value is hard-coded to 100, so we use that instead. This also makes it easier to set meters, imo. Eg if you want the meter to be half full, you set it to one half (0.5). I'm open to ways to simplify it, but I think this in particular is a capital-G Good idea.
The Math.clamp() part just keeps the value between two other values, in this case, 0 and 1. It's just a sanity check to make sure you don't pass in a bad value and create an error.