Transitional Credit On Excise Duty

Based On My understanding transitional credit for Those who are not registered in Excise But are registered in GST making supply of goods taxed @ 18% are entitled to claim credit upto 60% of tax paid Under GST if duty paid invoices are not available. Whether should I wait to fill TrAn-2 to take credit or shall I fill Tran-1 and take credit??? If the amount is to be shown in tran-1 part 7a what amount should i mention in eligible duties. Either the tax charged on invoice or 60% of the tax charged in invoice??