Hi Nigel
You need to be compliant with the UK HSE regs if this work is in the UK. The regulation is known as LOLER:-
http://www.hse.gov.uk/pubns/indg290.pdf
You will note that the certification as safe to lift has to be done by a competent person. As with much of the HSE regulation that has been interpreted by the courts and will probably mean a qualified member of one of the recognised engineering institutes with a specialism in stress calculation.
Calculation formulae can be found in 'Roark's Formulas for Stress and Strain' and in the Machinery's Handbook.
Stay safe
Regards
Pat