I think the full answer is
237.3/(1/(LN((HumidityPercentage)/100)/17.27+(AirTemperatureC)/((AirTemperatureC)+237.3))-1)
Easy way to get this result:
If in a spread sheet you put
air temperatureC in H7
relative humidity in H8
Then in H9 put the formula:
=237.3/(1/(LN(H8/100)/17.27+H7/(H7+237.3))-1)
This will give you Dew point
If you then put in H10 the formula:
=((H7-H9)/2.44)*1000
This will give you cloud base (well not far off)