Spreadsheets are Great
Whenever I'm unsure about a rule, I pull out a basic spreadsheet, and start populating numbers.
The image I had for the ghoul spell began with a mage who could raise small things from the dead, like a cat, or dog. Slowly, the dark sorcerer would learn to raise gnomes, then humans, and finally raise undead ogres or basilisks.
The system should clearly set some maximum HP for the target. Standard spells in BIND always use the caster's Intelligence Bonus, and sometimes use the spell's level, so the final formula would have to be some combination of one of those.
L3 + Int
With this entry, the caster can raise something from the dead with a maximum HP equal to the spell's level x 3.
The first entry looks good - 3 HP, which suggests a very small gnome, or a cat. A mage with Intelligence +2 suggests a very gifted necromancer, who might raise a dog, or even an elf from the dead.
At level 2, a mage with +1 Intelligence might raise an average human - but not a strong fighter. This seems okay, since the Ghoul spell doesn't need maintaining - ghouls just run around forever, and necromancers traditionally work with numbers rather than quality.
Moving to level 3, things still look sensible. A mage of this level should be expected to have Intelligence +2 or more, and would be able to raise most ogres.
However, at level 4, we have reached the peak too early. Almost nothing in Fenestra has 15 HP. Going to Level 5 won't help raise anything bigger, so it doesn't have much point.
Another problem here is that spells can be made Potent by adding +1 Level. Someone casting a Potent Ghast spell would raise someone with a maximum of 7 HP, not 6, so the spreadsheet here would have to be redone.
This sheet shows a maximum HP of 3 + the level + Intelligence.
We'll put add that Intelligence bonus in the brackets, so
3 (5) means that a level 3 spell can get a +2 Bonus to intelligence, effectively allowing the caster to raise 5 HP.
The first number here seems fine - cats, normal dogs (not hunting dogs), gnomes - but nothing too big. Now immediately going to the end, that '16' looks good. It suggests the limit for ghouls is a creature with +10 Strength, which seems like a fine limit. Top level necromancers with Intelligence +2 could raise a dragon, with +8 Strength.
Going back to Level 2 Necromancy, we find humans can be raised; and by Level 3, strong warriors can be raised. Alternatively, at level 3/ Intelligence +2, the necromancer could create a Wide, Potent ghoul calling. This might raise 4 normal human corpses from the dead, with a maximum of 7 HP.
Originally, I made about four or five spreadsheets. With nothing working, I stuck raw numbers into the cells, and retro-engineered the solution.
Rules which seem okay in the first couple of examples one thinks of can start to show their cracks once laid bare on a spreadsheet.