If you're using metric (millimetres) feeler gauges the gap should be no smaller than 0.2mm and no bigger than 0.3mm so if you have a 0.25mm feeler gauge then use that as its the mid-limit.

If you're using imperial (inches) feeler gauges the gap should be no smaller than 0.008" and no bigger than 0.012" so use a 0.010" gauge as that is the mid-limit.