It is very important to use standardized measurement as it is one is one of the six key elements of brewing good oolong tea. (The other five elements are: good water, high quality tea, right teapot, right water temperature and correct steeping time.) Remember that eyeballing a tea for measuring purposes can not be reliable.
Oolong tea with smaller leaves and tightly rolled oolongs are best for measuring with teaspoons. The teaspoon we mentioned here is not the teaspoon in your silverware set but a measuring teaspoon. Oolongs with larger leaves are suggested to be measured on a scale.
When trying a new tea, you may follow up the instruction that the manufacture gives you or you may also try it with different leaf amount and measure it on a gram scale. Through several tests, you will find the best amount for making the new tea well.
Related: Oolong Tea Preparation