I have a rule now that when designing a page, any "money screens" get at least 1.5X to 2X the estimate. I define a "money screen" as anything that leads a company to land a client or land a sale, things like checkout flows, signup flows, etc. Usually that extra time gets sucked up in A/B testing setup and setting up a staggered deployment per region that the biz operates.
Whenever customers push back I tell them the story of Knight Capital [1]. You pay extra for extra assurance that you won't loose a shit load of money in the future.
In the early 2000s I was a professional day trader and Knight was a market-making firm. EVERY single person I dealt with was an absolute crook, happy to break rules and do disgustingly horrible things to enrich themselves, because they were truly incompetent traders.
Whenever customers push back I tell them the story of Knight Capital [1]. You pay extra for extra assurance that you won't loose a shit load of money in the future.
[1]: https://www.henricodolfing.com/2019/06/project-failure-case-...