I'm programming a tool to help with monster building. I'm leaning a lot on the angry GM

The features Undead Fortitude and Relentless adjust HP by a static amount. As I am calculating defensive CR, I'd like to factor that HP bump into the CR calculation. Since HP at each tier of CR is given as a range, I'm curious how others would handle that calculation.

I'm sorry if I get some of the below numbers wrong, I don't have my books right now

A CR 4 monster has 116-132 HP. These traits above increase HP by 7[ish] at this level. Adding 7 to 116 HP is still a CR 4 monster. Adding 7 to 132 is a CR 5.

The only way I can think to resolve this is to decide starting defensive CR and monster size, calculate hit dice and what the average HP would be for that CR and that monster size, then apply the trait bonus and see if that HP increase bumps the monster into the next level of CR.

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    \$\begingroup\$ I'm having a hard time understanding the specific question you are asking and am unsure how to edit to better clarify. Can you relook at this and rephrase? I think you're asking about how to handle the CR bump that happens by the application of these features, but I'm not sure. \$\endgroup\$ – Pyrotechnical Mar 29 '17 at 12:36
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    \$\begingroup\$ Hi Nate, and welcome to RPG Stack Exchange. Right now your question reads to me as "if I add some HP in some circumstance, the CR might not get bumped up. I might solve that [unstated] problem this way, how would you do it?" The issue is there doesn't actually seem to be a problem or question here in anything you've described, so there's nothing for us to solve or answer -- sometimes extra HP just doesn't bump CR up, and that's how it's meant to work. What's the problem exactly? \$\endgroup\$ – doppelgreener Mar 29 '17 at 12:41
  • \$\begingroup\$ Here's the proposed edit I think you're referencing. I believe it was rejected since we couldn't be sure it was representative of your words and intentions, and editing your question to be asking something you're not interested in asking wouldn't have been appropriate and helpful. \$\endgroup\$ – doppelgreener Mar 30 '17 at 23:18
  • \$\begingroup\$ @doppelgreener I think my edit clarified what I'm asking. \$\endgroup\$ – Nate Anderson Mar 30 '17 at 23:55
  • \$\begingroup\$ @Nate Thanks for taking the time to edit that stuff. I've given it an edit -- does this reflect what you're going for? (If not, feel free to edit it further or roll it back.) \$\endgroup\$ – doppelgreener Mar 31 '17 at 0:01