Logic Statements

Hi 

I am looking for some help in regards to logic statements. I don't know if what I want is possible. 

I would like to be able to have an if/else statement that reads: 'if there is a numerical value present complete this calculation (=(((H9/100)/0.00785)*385)/(G9*1000)) ELSE if the value is 'n.d.' calculated value should be zero.'

Can I do that? 

Thanks!