IF AND Statement

I have the following If statement which is working the way I want it too.

However I need to add an extra condition where IF C7 has no data in it then it will show as 0. At the moment the cell the formula is in is showing #VALUE.

I think that I will need to use IF(AND however my attempts have failed me.

=IF(C6="BT",12.88+(C7-30)*0.6,IF(C7<35,4.3,4.3+(C7-35)*0.2))

Thanks