Hi ArosP. We used to use the Marriages premix with Flubenvet -so easy. A 10Kg bag will just be enough for your hens I think. No treats during the 7 day period.
If you do get the Flubenvet powder and mix your own be aware that the small (60g) tub mixes one scoop (6g) to 2Kg of feed, so you have enough for 20Kg of feed. You will probably find the premix works out cheaper, even with postage.
A large Flubenvet tub is a more concentrated version and a 6g scoop (they don't supply a scoop with the larger tub though) mixes with 5Kg of feed. That is probably cheaper than the premix, but it's awkward mixing 5Kg in one go. We add a little olive oil to the powder first so it sticks to the pellets better.