With regard to the quantity of feed, I have never tube fed a bird myself, but this is a very detailed explanation:
https://www.backyardchickens.com/t/...h-my-bird-pics-for-visuals-very-detailed-post
Kuntrygirl says she started with 4 oz (120ml) of food, then went up to 6 oz (180ml) - twice a day, so I think you could quite easily increase the amount you are giving your girl.
https://www.backyardchickens.com/t/...h-my-bird-pics-for-visuals-very-detailed-post
Kuntrygirl says she started with 4 oz (120ml) of food, then went up to 6 oz (180ml) - twice a day, so I think you could quite easily increase the amount you are giving your girl.