Bearing in mind that I have zero formal training in reliability engineering, my suspicion is that if it’s going to be subjected to mechanical abuse, then you’ll need to do testing to find out where your weak points are.
For sure, the effect of board flex depends massively on a) what’s on the board (a PCB with just some ICs and small chip resistors and caps will fare very differently from one with a big transformer on it), and b) what non-shock forces it’s subjected to in use (for example, a board with buttons or external connectors on it needs more support than a purely internal board, since every button press and connector insertion causes flex).
Flex aside, mechanical shock can cause SMD components with a high weight-to-pad-size ratio to be torn off the board; a classic example of this is larger inductors.
Thinking aloud here: depending on the types of shock, maybe a compliant shock mounting would help? Something like mounting the board with rubber grommets.
The upshot being that I doubt there’s any simple formula you can use. Maybe there exists software that can perform such physical simulations?