13 grams is pretty tiny. But you would give her .1 cc, which is about 3 drops. But if she's already twice that weight, you're gonna have to give her .25 cc.
You should really invest in a digital scale just so you can be sure you're giving her the right amount. If she's small, then it's especially critical to get a proper dose. They sell for about $20-$30 at an office supply store (in the shipping/mailing aisle). A scale will come in handy. And you really need to weigh your gecko before you administer any Panacur.
I give them medicine by just putting dabs on their mouth and letting them lick it up at their own pace. It takes a firm grip to hold the leo still. It's something you get better at doing with practice.
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chickabowwow

3.2.3 leopard geckos (Rosie, Locke, Lisa, Caesar, Tommy)