Looking for an accurate scale to weigh medicine

Can anyone give me advice on a decent scale that's not a bank breaker?

I run a state licensed out of home wildlife Rehabilitation center and often deal with small mammals that require medications for various illnesses and injuries.

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The problem is alot of the medicines we get are scaled up for large mammals or humans and have to be broken down and measured out to get the appropriate sized dosage for say a baby opossum.

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Example: Clavamox 1gram of powdered medication is equal to a 200mg dose

The dosage for opossums is 20mg per Kg of opossum and I have a possum that weighs 90grams so basically 1/10 of that 20mg dose is needed to medicate

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So now I need to find a scale that can accurately or at least close to accurately with out being professional lab grade measure out the small amount needed to medicate the animal.

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I have tried using one of those small (drug dealer) style pocket scales but they hardly read the weight of the medicine unless it's over a gram and at that point I doubt the accuracy.

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Any suggestions?