The real problem is that the civilian performance charts don't tell you the information you need to know.
In this case, you need to know two things - power required to hover, and power available. VEMD and the power check chart in the FM can give you power available, but the 'hover capability' chart doesn't tell you the power required to hover. (I gather there may be one or two helicopters that do, but the majority do not have a power required to hover chart).
So unless you know a good rule of thumb for how much more power will be required for the additional weight, you're into the land of guessing.
If you know how much additional power is required to hover OGE vs. IGE then you can do a pretty good estimate once you load the additional weight and hover IGE in the hole.