MHP works for NPCs as of yesterday's update.
That being said, definitely check out templates if you are spending much time on your approach, they work great :)
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So, i bought the Advanced Bestiary and although i'll probably use some of it, it isn't the most adequate for my uses since it can't add ATK or DMG bonuses to NPCs.
That said, i'm having a bit of trouble using 1.25 version of Live Hitpoints as of now. I can only get CON or MHP bonuses to work with NPCs when they are active on the combat tracker and their respective combat tracker character sheet is open. PCs CON:N and MHP:N bonuses work fine, without the need to open the character sheet. I don't know if this a limitation of fantasy grounds with this extension.
Here's an example:
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Attachment 39113
Hey Zygmunt, this is actually Pathfinder. I don't know where you got the 5e from.
Yeah, there are some fields for attacks on the template sheet, but attacks always need some manual adjustment with that; the problem here is probably the simplistic structure of NPC weapon attacks and damages, such that FG does not really know which ability etc. is used for these strings and so the automation for that is a bit difficult without causing more trouble :)
Yes, it's really not a very useful effect :) but, apparently useful for templates (I'm glad adding it was not a waste).
Oops, I shouldn't have tied that script to the UI. I'll get that fixed for the next update, hopefully.
EDIT: I have a fix working, will be releasing with tomorrow's batch of updates.
I was updating one of my campaigns to use this extension and one of my characters ended up with an extra +1/lvl that I couldn't account for. After getting frustrated and trying to figure it out, I finally shut down the campaign and looked at the XML. There was an element added to the DB called "livehpused" which doesn't appear to be exposed in the UI. It is a very very bad idea to have modifiers to HP that can only be changed by opening up the XML and making manual edits. Whatever code is adding this element, I suggest you remove it. The entire HP calculation should obvious to the player, and total HP should be adjustable by modifying the associated stat and the HD HP edit box. No more, no less.
EDIT: FYI, this is a character that is using Cha instead of Con, if that matters.
EDIT2: Hmm, I set livehpused to 0 and my character is still getting an extra +1/level from somewhere. Attaching a picture to illustrate:
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Character is level 13. Note that the Cha bonus is +9, but Abil. HP result is 10*lvl.
Don't worry, livehpused is just a check value that gets set to 1 if a user has used the extension. This is so that in a future version I will be able to automate the process for new users (so they don't need to figure out their HDHP manually).
The only thing like like you describe which I can think of is the NPC's 'misc' hitpoint modifier. When a pre-gen NPC is added to the combat tracker, it calculates what the ability-derived HP should be and compares that to the fixed number in the HD field. The difference between these is saved as a fixed value that is applied on top of everything else to keep it from altering the creature's base HP. That being said, you can actually adjust this without using xml as changing the fixed HD value and then clearing the HDHP field will repopulate the misc value (although this would be in no way apparent to anyone not reading the code).
Any chance the character has toughness? If so, this is automatically added so if it has been added manually then it would seem to be artificially high. I have now added this to the readme as I had forgotten before so it was poorly documented.
Ok, it was toughness.
However, I have a different character who added toughness to their feats pane, but then changed the name to be prefixed by the level at which they acquired it (i.e. "15 - Toughness", and your extension does not pick that up. I renamed the feat to verify. If there are other feats out there with "toughness" as part of the name, then changing your logic to be a substring match is probably not the right thing to do. It's not a big deal because they added it manually to the HD HP box, but just thought you'd like to know.