I have just started riding pickle in a grackle, it can't be loose or it just slips round their face! this is the best picture I can find of him wearing it (anyone can correct me if they think it is incorrectly fitted)
It depends on the type of grackle as to how it should be fitted. I have a mexican grackle fitted like this,
but the type without the metal ring on the cheek are fitted lower. I fit mine quite tight, but can still fit a finger in the noseband. I was told it wa the pressure it gives when they cross jaw/open mouth, rather than straping their mouths shut!
Over here grackles are Dressage Ireland legal, so I use a grackle and a single-jointed eggbutt for dressage with Ali - he's a very strong horse but I still have pretty decent control with a grackle
BTW, I recommend that you buy a high ring Mexican grackle instead of an "ordinary" grackle, as they tend to slip down. Here's a photo of how I fit Ali's Mexican grackle -
Mexican grackle gets my vote too, works really well for horses that open their mouth or cross the jaw to avoid the bit but I prefer it to a drop or flash because they seem to irritate them less and not interfere with their nostrils.
This is the "ordinary" grackle. It did slip like crazy on my dales. But on my arab it stays still (so far). The mexican grackles are way better than this cheap-o version! I am currently saving for one.