To some degree, you do have some wiggle room. This also varies I think with the particular bacteria in question.
Precise amounts don't always matter. For one thing you don't know how viable your culture is at any given point and another thing is the way bacteriae multiply exponentially. So if you start with a smaller population, that population will, let's say, double in x amount of time. In the same amount of time, the new population will double again, and again, and again. Exponential growth curve until they reach certain environmental thresholds (limited by temperature, food source, chemical conditions, pH, competing bacteria, etc.)
If the growth rate was direct (that is, x population of bacteria will be produced every minute under these conditions) than culture amounts would be a lot more important.
So if we start with an amount of viable, active bacteria within a certain range (reflected by your 1/8 to 1/2 tsp measures) then the time factor will vary only slightly.
This, of course, has its limits as well. Obviously if we way undershoot the necessary amounts we can run into problems (especially secondary inoculations) and if we overshoot the amounts we run into another set of problems, especially runaway acidification. And then when it comes to mixing multiple cultures together to make a certain profile, we need to maintain ratios within certain limits or things won't work right.
The biggest advice is, don't sweat the apparent inconsistencies. But, this reminds us to be careful to observe the cheese and don't just trust that things will always happen within a certain amount of time.