You always charge hourly, it’s just that you don’t always expose it as such. All of your costs are either driven by time (rent, salry and benefits, SaaS costs, calories per day) or amortizable over expected number of billing hours. You need to know your total loaded cost per billable employee, because you’ll need to charge more than that for an hour of their time. In the end, fixed-price contracts are just really crappy T&M contracts from an estimations perspective.