Darth Sidious' Health

The increase in max health from his unique is applied at the start of battle and doesn't necessarily apply after the leadership ability has added the 40% potency. I don't know if this answers your question as the numbers still don't add up exactly...

If he has a potency of 123% + 50% (from his own unique) = 173% then his max health will be 26094 x 2.73 = 71236. So 20% of that would be 14247.

As I said, not exactly right with your numbers but closer. Was there any other max health gains involved from another mechanic or buff? The 495 discrepancy multiplied by 5 is a 2475 discrepancy in max health, which could be explained as approximately 10% of 26094 if there's an additional 10% max health added from another ability or buff?

Alternatively, the 14247 I calculated could indicate a transcribing error since you say you observed 14724?

Hope something there gets you closer to the answer?

Yes, you're right I made a transcribing error, 14.247 is the correct value.

There was no other potency or health gain involved, but I'm sure your solution is correct.
The differences probably stem from different value calculations ingame.
Like you said, that the starting potency is additive and the added potency from effects is multiplicative.

Thanks a lot! :)