Gary,

 

Walter sent a formula (courtesy Advanced Pilot Seminars) that I have built into a spreadsheet.  It is for % power based on RPM and MAP when running ROP.

 

See the attached Excel Spreadsheet.  Fill in any max RPM and Max MAP.  I used the MAP I read before engine start corrected down to sea level, 29.9.  Also, fill in any rise (inches of Hg) due to Ram Air developed from the max IAS to calculate the highest possible MAP used in the chart.