you should be able to fit two fingers between the flash and the horses nose, the flash should only come into play when the horse opens his mouth, it should not be tight all the time.  hope this helps xxx

Thankyou that really helped because i just need to keep my horse under a bit more control when schooling sometimes because he can be a bit strong x

A two finger tightness.

Don't have it so tight that your horses mouth can't move at all as this can restrict breathing.The flash is only designed to keep the mouth closed to avoid the tongue coming over the bit and for controll. STick to the basic rule and youll be fine. :D