HP Formula for Insubstantial monsters?

[Rechan]

I notice that monsters who have Insubstantial have lower HP than they should for their level. This makes sense, because they effectively have twice the HP.

The thing is, I can't figure out the formula that is used to determine how these numbers are totaled.

Anyone have a rule of thumb?

 

[Jack99]

-20% ballpark seems to fit quite well.

 

[Flazzy]

-2/role usually works for insubstantial and regenerating monsters (brute becomes 8/level+1, soldier 6/level+1, lurker 4/level+1 etc.)