well i've not actually started my career - but isnt it the base of the rule of thumb that the cloudbase should be about 400 multiplied by the difference between temperature and dew point?
Ive got only vague memories (worrying since I sat met 3 months ago!), but its 400 feet (ie 1000 / 2.5) because the SALR is actually a curve not linear, so 2.5 is a better estimation that 3.
Of course I could be talking rubbish!