Find your real cost drivers
Hours are a result, not a driver. The things that make one client take longer than another are measurable: number of bank transactions, number of bank accounts and currencies, number of sales invoices and supplier bills, inventory, payroll, and whether the client is VAT registered with monthly or quarterly returns.
Pull a year of data for twenty clients and see which drivers correlate with time spent. In most UAE practices, bank transactions and currencies explain most of the difference, with inventory a close third.