Given that precise EV calculations require one to operate with rather uncomfortable numbers, approximations are used along with simplified models.
For instance, the rule of 4 and 2 allows you to convert outs into % probability, which could then be used to estimate a profitable bet size with reasonable accuracy.
Are there any other similar rules that I've been missing out on? The intention is to be able to estimate odds and derived ROI more accurately, and one of the areas that imo still requires "proper" maths is doing so relative to opponents' stack sizes.