You do not say which model of 150 but go here, find it and open one (Or Yours)
CAA issued Flight Manual Change Sheets and Supplements Search Results Summary | Airworthiness | Operations and Safety
These are the CAA additionals for the 150 range, you should find the write down figures in those, example below, they should be in your Flight Manual BTW
The write down relates to in the good old days a manufacturer would put out figures that never related to the real world, shiny new aircraft, new prop new engine, ideal conditions, test pilot etc, after all that's what sold your aircraft, you do not see a cigarette company selling their product on the basis that they will kill you, same with aircraft, you would never sell it on the basis that it was slower than it needed to be.
Example
http://www.caa.co.uk/docs/DecOfFligh...UP_1_ISS_2.pdf
Cessna Type Data certificate for the 150 is here
http://rgl.faa.gov/Regulatory_and_Gu...20Rev%2048.pdf
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