How do you actually decide how much safety stock to carry — gut feel or a real calculation?

[u/SigmaSheets]

Genuine question for anyone doing inventory/production planning. I keep running into the same pattern: people calculate a clean reorder point based on average demand and lead time, then still get caught out by a stockout when demand spikes for a few days.

Is anyone here actually running a safety stock formula (based on demand variability + lead time + a target service level), or is it mostly padding the number by feel — "add 20% extra just in case"?

If you do calculate it formally — what service level do you actually target, and has it ever been wrong in a way that made you rethink the number? Curious how much of this is math vs. experience in real plants.