Most customers use our Compass for Pressure or CalTool programs to run calibrations of several of our products (Ex. 6270A, PPC4, RPM4, etc.) and to calculate and/or apply the these, but if don't have this software or want to calculate the adder (PA = Pressure Adder) and the multiplier (PM = Pressure Multiplier) manually and collect the data by other means this is fairly easy.
The adder/multiplier is a standard slope intercept calculation of y = m(x) + b. Easiest way to calculate this is Excel with LINEST formula. Here are some instructions for this: http://www.ehow.com/how_7297515_use-linest-function-vista-excel.html
See the above example of data collected and how the Excel formula was applied. If you then look in the Function Arguments Box you will see it has calculated the multiplier (PM) as .936 and adder (PA) as 138.592 . This calculation yields the same results as COMPASS or similar calculation and can be manually entered into our products that use this type of calibration coefficients.
We also still offer the CalTool for RPTs software for free which can also gather the data and apply the coefficients to some of the older products (PPC, RPM, etc.). It can be downloaded here: https://us.flukecal.com/literature/software-downloads/free-utilities/caltool-rpts-free-utility