Ignoring the "brewing instructions" is almost always good advice. I think FlyedPiper's advice is more or less sound.
One thing I'd add is that the quality of the tea affects how much heat and leaf quantity it can take. Try brewing a tea with a pretty heavy hand the first time, and you can always adjust parameters downward the next time once you know the tea well.
Also, even though technically boiling is always boiling (i.e., 212 F / 100 C), there are a lot of variables that affect how hot the water actually is when it hits the tea. I alluded to this in another recent thread.
For rolled teas like tie guan yin or taiwanese oolongs fill the vessel about 25% full of leaf. For other teas like yancha or dancong that don't expand as much fill the vessel 50% full.
Good advice. But do keep in mind that most mainland tieguanyin and some competition grade Taiwanese teas are 1-leaf, whereas a lot of hand-picked Taiwanese rolled oolongs are two leaves and a stem. The latter will expand much more.
Also, keep longer-leaf wiry teas which are unbroken will need to be packed in more; smaller leaf and / or broken wiry teas may benefit from a little more moderation in quantity.
While I'm not a big advocate for using a scale for every day drinking, it can be helpful when brewing a sample or an unfamiliar tea.
Longjing can be a pretty mild flavored tea. But it can be brewed with more tea leaf, just adjusting it to your taste. If it's exceptionally good quality (unlikely), you can try a bit more heat and see how you like the results. I don't brew green teas often, but I would probably use a gaiwan and maybe 1/4 full of leaf by volume. Some people say that you should leave a "root" when brewing Chinese greens, i.e., don't drain the tea completely when pouring the tea off. You can also try leaving the brewed tea in the gaiwan and drinking straight from the gaiwan, which is a very appropriate way to drink longjing.