At a guess only -
The 2 numbers you are quoting are the TO ratings of the engine not the MGB.
From memory only there is no difference between MCP and TO power in a B or a BA. 83 and 88 % resp i.e you can fly it all day at TO power - other limitations being considered of course. This equals 532 HP on a B and 564 HP on a BA.
Also in a B2 there is a limitation on MCP and TO power versus speed i.e TO power may only be used up to ~ 40 knots. It probably equates to only being able to use the extra power over a BA in the hover.
The other issue you may have is that the Q meter indications i.e. % versus pressure from the torquemeter may be different on a 1B engine and a 1D1.
Also keep in mind Q for a constant RPM versus collective pitch angle is non linear.
If your question is in response to the NZ CAA slingload with pax operations the "power margin" is virtually impossible to work out with the data you have available. You could be Ng limited - how would you work that out? It would have been much easier if they had imposed a weight margin from the published performance data.