Unless your birds freerange and get a lot of their food from their range I'd medicate their food which MUST be homogeneous (consistent composition) and ALL treated uniformly not just one component so they don't pickout the component they like and leave the others they don't like. It must be given in a ad hoc feeding system so they eat fully in proportion to their needs and aren't deprived by the more dominant hens.
This is the problem with grape feeding as well as ensuring an accurate measure. If you feed medicated grapes it's only possible to do with a small number of hens you can identify and hand feed individually. If you have several hens and you attempt to feed them together the ones at the top of the order will get more grapes and be overdosed and those at the bottom will be underdosed. Problem is if one bird takes the grape the others will try to get it off her and you can never be sure which ones have been medicated and which haven't.
Incidentally I calculate a daily dose of Flubenvet 1% per adult hybrid layer to be around ) 0.4 of a gram/day. My calculation is as follows. Each hen will consume about 125grams/day (or 1 eighth of a kilo) and the dose rate for the feed is 3grams /kilogram of feed so each hen would need 3/8ths of a gram = 0.375 grams. 0.5grams would be OK as Flubenvet has been tested to be safe upto at least 2.5X overdose.
Below is an illustration of what happens if you mix ingredients in a single feeder.
I had a couple of handfuls of corn left over after their evening feed so rather than retun it to the feed bin I put it in their feeder which had pellets in. The next morning the clever little so and so's picked out all the pellets to get the corn. This illustrates what can happen if you mix ingredients in a single feeder. The pellets had been medicated but the corn of course hadn't.
HF