Chickens aren't dogs and it really depends on the product you are using as to the actual dose you give. Granules, 10% or 2.5% suspension, Pastes, Tablets...there's lots of choices of Fenbendazole on the market. But generally for dogs the dose is 50mg/kg, you just have to figure the dose depending on what product you choose to use
The most Common form of Fenbendazole that people use for deworming Poultry is Safeguard Liquid Goat Dewomer (10%) or Safeguard Equine Paste (10%). Dosing is 10-50mg/kg for Poultry, but most commonly 50mg/kg is used, hence the 0.23ml per pound of weight.

Re-Read the instructions I posted at
POST#2
For Roundworms ONLY, give once, then REPEAT in 10 days.
For other types of worms, give for 5 days in a row....