I would always run the numbers to see if it is possible, and then apply safety margins and finally have my own input on how I think the plane will behave.
The safety sense leaflet shows how the calculations ought to be done:
safet sense leaflet pdf and there is a link to AIC 127/2006
AIC for exact figures to be used.
From memory this was the required factors for the exam, but something to note in real world flying is when the aeroplane's POH was published and/or amended to check whether or not they already included the CAA 30% safety margin factor.
I hope this helps!
Alex