before landing check list,
Do you are saying it is correct to use the single engine service ceiling chart ad a drift down chart?
Not exactly no. I have no experience in the King Air and don't know what charts you have available for en-route performance in the AFM. What you said was:
the King Air manual has a Single engine service ceiling chart and people are using as drift down.They are entering the chart at OAT that is derived from flight level temps and using those figures as opposed the OAT temps on the ground at takeoff, resulted numbers are much different to say the least. I say there are no published drift down charts published for this aircraft and the chart is used incorrectly if using FL OATs. Which is correct please?
...and what I am saying is:
They are correct to use the temps aloft.
What I mean is that the en-route OEI performance should be based upon the conditions you would expect to encounter at altitude along the route rather than extrapolating some surface observation using a standard lapse rate.
You say there are no drfit down charts published. But for a commuter category aircraft there must be a chart that allows you to determine the OEI en-route climb performance. It is not specified exactly what format the chart must take, only that you must be able to determine the climb performance given the weight, altitude and ambient temperature. That is neither the "service ceiling" nor the absolute ceiling, but it may be that all those things can be determined from the same chart.
The Pacific and General Aviation section of the forum seems to have a few people experienced with the King Air who could probably tell you exactly what chart to use for drift down if you post this question there.