Buy a set of measuring spoons...

For 4 adult Rhacs (one crested gecko, three gargoyle geckos) I mix one teaspoon of powder to two teaspoons of water. I divide it among the four geckos...so probably 3/4 teaspoon each. Adjust by how much is left in the bowl..if the lick the feed tray clean add a bit more.

Basically when mixed it should be the consistency of milk.. so even without accurate measuring, you can guess how much water to add.

I typically make batches that are actually more like 3 teaspoons powder to 6 teaspoons water, then refrigerate for up to a week...so that feeds my geckos for a week's period...I feed 2-3 times a week.
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PHLdyPayne