I was thinking about how to improve my fermentation procedure and reach a consistent yield of a full 5 gal of finished product after fermenting in 5 gal corny kegs. Searching on the net I've found this pic, and think: there is a heigth point to put the airlock that could entirely avoid blowing off? The plan was to fill the keg to the brim, link a long hose to an airlock and hang it high. After fermenting I could manage trub losses and keep a full serving keg. That question makes any sense? The diameter of the hose or the keg fittings could add anything to this?
If fermenting in a corney keg you will never yield 5 gallons of finished beer.
If 5 gallons finished beer is the goal use 6-6.5 carboys and brew 5.5-6 gallons of beer.
Primary yeast cake easily takes 1/3 gallon, secondary can be a little less.
Allow a good 1/2 gallon loss of dry hopping.
Headspace is how you will limit blow off loss. If filling a corney with 5g and using an extended airlock as in your pic you will make mess unless it's a style of beer that has almost no head and a really slow fermentation.
That airlock imaged would only work for about a 4 gallon batch, with a final yield of about 3.5 gallons.