I'm a newbie so consider that as you read my post.
From what I hear, 1 oz ppo is a reasonable upper limit for CP soap, with 1/2 to 3/4 ppo being intermediate values. My limited experience seems to agree with these numbers.
I hear that HP and rebatched (hand milled) soap needs less scent because the EO/FO isn't added until after the lye has been completely reacted and the soap is neutral, so the EO is not as eaten up by your lye as with CP soap.
So based upon that, and recognizing that this is just conjecture on my part, I suggest that you might consider 1/2 oz ppo as a reasonable limit for HP, rebatched or milled soap. If you get no better answer than my conjecture you might try a smaller test batch and find out how 1/2 oz ppo works for you.
It would really depend on the essential oil that was being added. Some are much stronger and should be used at lower volumes....ie. eucalyptus I generally use at lower volumes....0.5 oz PPO. Whereas citruses I would use @0.7 oz PPO.
Frangrance oils depend on how strong they are. I generally use my nose for FOs.