Answer specific to your question:
If ( not IsEmpty (Taxable) ; //put taxable expression here ; //put nontaxable expression here )
An improvement to consider: replace 0.625 with a field so that you don't have to rewrite your calculation everytime the goverment raises the sales tax. Instead, you can simply enter a new tax rate in the field. This can be a field that looks up the value from a related table or auto-enters the contents of a global field.