I hope this doesn't confuse things....
You need to give 25 mg of fenbendazole per pound of body weight. So for the 10% solution you give 1/4 ml per lb, 1/4 or .25 ml = 25 mg in that solution. With the 200 mg/ml solution you would give 1/8 of a ml to get 25 mg. Depending on the weights of your birds, if they are very small, you may need to convert all that to grams to get a more accurate dose, it just depends on how small they are. So once you have their weights you can figure how much to draw of the medication, and then I would add that much again water to it to dilute it by half, which will increase the volume but not the dose strength. It will still contain exactly the same mgs of medication you drew up. But the increased volume will make it a bit easier to give, and may make it a bit more palatable (I've no idea how it tastes). That is what I would do, I hope that makes sense and doesn't confuse things.