It's not exactly fair.. and also not likely without extensive time spent doing the estimation, like sure - you can be more precise, but the more precise you need to be, the longer it takes you to frame the estimate.

Two books help with methods of doing this.

Rapid Development: Taming Wild Software Schedules: https://www.amazon.com/Rapid-Development-Taming-Software-Sch...

Software Estimation: Demystifying the Black Art https://www.amazon.com/Software-Estimation-Demystifying-Deve...

This likely does mean you'll need to be allowed to deploy automated test cases, develop documentation and the entire nine yards, because it's going to be far harder to hit any reasonable deadline if you don't have a known development pipeline.

You'll need to reduce your uncertainty as much as you can to even have a chance - and even then, things will still blindside you and double (or more) the actual vs. the original estimate.