I am currently working on a mod to do exactly this (and a lot more), for OpenMW. It DOES take into account the starting bonus and only adds it to determine the initial health, then uses the correct formula to determine the bonus health on top of the initial, so it should be perfectly predictable at higher levels - if you have 100 Endurance and had equal starting Strength with 2 characters, you'll have exactly the same health, regardless of when you levelled up your Endurance, and the numbers are very similar to vanilla (usually within about 5 total HP). This does constitute a nerf to The Lady birthsign, which isn't so insanely broken anymore. Obviously having 125 Endurance instead of 100 Endurance will give you more health, so it's still a useful buff throughout the whole game, but doesn't give permanent health boosts anymore. Instead, it's just like any other Fortify enchantment. Good, but not godlike. This system currently works fine, but can't account for custom classes. There is an existing PR for OpenMW to add support for reading data from NPC classes, which should let me read their Attribute Bonuses and correctly calculate the players starting health bonus. When that's merged, I will be ready to release this. Feel free to PM me if you want to follow progress with this.