How to handle grid system and layout at different breakpoints when designing wireframes / mockups?

So I've researched this question and I've alluded to an answer, however I'd like some more input. As a developer whenever designing a website I could just code as I go and set breakpoints whenever something doesn't fit. However switching over to UI design I'm facing the issue of compiling the proper screen sizes / mockups to hand over to developers. Since there is essentially no way to determine breakpoints for every screen when purely designing mockups what exactly is best practice here in this situation?

It would seem the best case is to make designs in a few predetermined screen sizes such as a mobile size / tablet / laptop / and large desktop. Since these layouts cover MOST of the ground, do devs usually just tweak the design a small bit when something doesn't fit?