Depending upon the format of the POH, I would expect that you are being provided with the %HP for the power setting stated on the chart, at the stated altitude, rather than having to flight plan to it. Other than referring to the cruise performance chart, you as the pilot have no way of measuring %HP in flight.
%HP is not something that you really need to know for flight planning, other than for fuel consumption, which is generally also provided in the cruise performance charts.
If this has not answered your question, perhaps a more specific question, with an example from your POH, will make an even more specific answer possible.