Eric, when the zip loc baggies are filled, I fill them so that when they are sitting flat that they are not bulging. If they are bulging and you cut a slit in them, then the sugar syrup will pour out, and that is not a good thing. So, in answer to your question, I don't fill them rightly full, yes, perhaps 3/4 full would be the good description. The baggies should look slightly humped, not really flat looking, or else then too much space within the baggie is wasted.
Linda, you say that the only problem with the baggie feeder is that you can't see when the baggies are empty. You must be putting them on the frame with the inner cover on top? (that is a question, BTW). Yes, that could be a problem to not be able to see how much is being consumed, that is why I put mine on the top of the inner cover, a shim houses this area so there is room for the bees to come up and drink and then the lid goes on the top of the shim. Linda, did you look at my pictures in the thread that I put the link to?
When I feed the bees with the feeder bag, I can see exactly how much is being consumed. That is because I place the baggie on top of the inner cover. The inner cover has a hole in it with a plug. I remove the plug and push it a little to the side so that only a few bees can get up at a time.
When I got my inner covers none of the had any holes pre cut into them. I asked my Husband to cut a hole on the end of the inner cover so that I could fit the baggies (one gallon ones) on top of the end of the inner cover. I then fastened a piece of wood that I nailed onto that plug so I could lift it off the hole easily. Hoping this little bit of further information helps more. Have that beautiful day in this great life. Cindi