Ins Portugal

I don't know if it's too naive to say this, but what confuses me is how you calculate the amount of wasted exposed film - I mean, not actual usable footage, but the time between the camera starting to roll and the actual beginning of the action, as well as the called 'cut' and the actual camera stopping. If this is included in the ratio, how do you roughly calculate how much you'll be wasting? If this could be residual when shooting a short length project, I can imagine that might not be the case in a feature, specially when shooting at a high ratio.