Try this...
=(B6-B2)*((15-(B8+(0.00198*B2)))/((273+(B8+(0.00198*B2)))-(0.5*0.00198*((B6-B2)+B2))))
Where: B6 is aircraft alt (ft)
B2 is aerodrome elevation (ft)
B8 is aerodrome temperature
The formula is copied straight from my spreadsheet and should work if the correct values are assigned. The formula is an older one but should work for your purposes.
Alpha