As a founder it sounds like a smart move to know what it’s going to cost you.
The problem is it ignores the reality of software development, which is one of constant discovery and unknown unknowns.
It may “never” be done.
It creates a misalignment of objectives.
The cost risk is not mitigated, just moved onto the developers, who will cut corners if they need too.
Unless you’re willing to pay a huge premium for a project based on value and not work/time (and most people are not).
A far better option for you is to truly partner with your developers, figure out how you can ship value as quickly as possible and monitor for acceptable progress.
And of course get rid of development teams that are underperforming and not meeting your expectations.