The most critical part of any print is its first layer, this is the contact point between the rest of the print and the print platform.

If printed too high then the model will detach mid-print, too low and will be a pain to remove form the platform.

What we want is a happy medium like this.

Too high - the first layer isn't sticking or the model has peeled off.

Too low - the nozzles is pressing the plastic down too much, or at worst digging into the bed.

If you find your first layer to be too low or high, stop the print and adjust the z-offset, as shown in next step.