Isd invoice

Dear Sir/Madam, Our branch is located in Cochin for which we do not take ISD registration. Further all accounts are handled by HO in Mumbai so for utilizing GST input credit of Cochin in Mumbai we raise sales invoice from Cochin on Mumbai and File GSTR 1 for Cochin. By this procedure we availed Input GST credit of Cochin in Mumbai. Further, is this procedure is correct? How to calculate Taxable amount in that invoice? Please guide in this.