Why a spoon misleads
A teaspoon measures volume, and teas of the same weight occupy wildly different volumes. A tightly rolled oolong is dense and a spoonful may be six grams; a fluffy white tea of the same spoon size may be one gram. Following the same 'one teaspoon per cup' advice across both gives you six times the leaf in one case. This is the single reason the same instruction produces good tea and undrinkable tea depending on the packet.
Rough equivalents worth internalising
For a 250 ml mug: broken-leaf black tea, about one level teaspoon. Whole-leaf black or green, a heaped teaspoon. Fluffy white tea or large dried flowers, two to three heaped teaspoons or a loose handful. Rolled oolong or compressed dark tea, a scant teaspoon. These are starting points to adjust, and adjusting is expected.
The better habit
Look at the dry leaf and estimate how much space it will need wet. Dense and rolled means less; light and airy means more. After a week or two of paying attention this becomes automatic and is more useful than a scale, because it transfers to teas you have never met.
If you do buy a scale
A 0.1 g jeweller’s scale costs very little and removes the guesswork entirely. It is most worth it for gongfu brewing, where leaf-to-water ratios are high and small errors show up immediately. For a mug at work it is overkill, and the estimate above is fine.