This can be a touchy subject. Some say never ever do it, some say it's never an issue. And maybe that plays into what you are feeding, as well as many other environmental conditions.
The first question with open feeding is whether or not you need to. If there is a good honey flow on, then you do NOT. In fact, I've tried open feeding pure honey during a natural honey flow, and the bees completely ignored it. If you are unsure about a flow, just do a simple test, offering out a little bit of sugar water or honey in a saucer and see how the bees respond.
So you do the test and the bees go bonkers for the feed. Then you want to feed more. Well, feed what? And if sugar water, what solution? You'll read about 1:1, 2:1, 3:2, etc. as a sugar:water ratio. Then again, is that based on weight or volume? I did a little test and made this video discussing what I found.
In general, I think the key controlling issue in the ratio of sugar to water is simply the ambient outside temps. If it's getting cold out, you really want to reduce the amount of moisture in the feed. The majority of moisture in the feed has to be extracted back out by the bees anyway, and in cold conditions can condense on the ceiling of the hive, dripping back on the bees and chilling/freezing them.
When it comes to how much to feed, I'd recommend not more than the bees can take in just a couple days. Mixed sugar water can easily get contaminated and start to ferment. If it does, the bees won't take it, and it just goes to waste.
Now what about honey? Do you have excess honey that you can actually feed back? This is my favorite, as it takes out all the questions of what ratios to mix, and the risk of contamination by the bees at the feeder leading to fermentation.
In my particular situation, I often have buckets of excess honey comb from cutout jobs I perform. I don't crush/strain/bottle this honey, but rather offer it back to the bees. They have a natural filter in their honey stomach that is far better than any sieve I own anyway. Not to mention, I'd just as soon let them do the work. I will say, feeding unknown honey is NOT IDEAL due to the risk of transferring diseases such as AFB. But I guess for now I've gotten lucky and in general AFB has been eliminated through good beekeeping practice and genetics. Also a quick note, if I know the bees have been sprayed with any pesticide, I NEVER feed that honey back. All of the honey and comb get's black bagged at the job and disposed of. Shameful waste, but better than feeding it to all my bees for sure!
The biggest issue I've had with open feeding honey, is the risk of induced robbing. The latter half of 2014 was plagued by colonies getting robbed in my own back yard, and for a long time I couldn't figure out why. It only started after the honey flow was over, but once it started it didn't stop. Until I stopped open feeding cutout honey...What I finally realized was happening, was that the bees would eat up all the open fed honey, and then go for the next best thing, the colony it came from that I had just brought home. This weakened colony (from the stress of cutout) easily succumbed to the robbing.
I'm still open feeding cutout honey, but I've changed my practices a bit to mitigate the robbing risk, and it seems to be working. I'll have to discuss that in another post.
In general if you are going to open feed honey in the comb, do so in an area that gets decent sunlight, but also has some weather covering to keep rain out. The bees will be so hooked on the honey, that if a chill or a rain blows in, or it gets dark out, they may just stay with the honey comb. If that in a bucket, and say it rains about 3" in an hour (like Houston likes to do) you just drowned a lot of bees! Only open feed comb honey in shallow containers, with a perforated bottom that can drain (if your feeder is not covered by some sort of roof). And just as quick as those combs are empty, melt them down, or put them in some sort of protected storage to keep wax moths out.