In order to fit well, a properly constructed bosal will have a short noseband. This results in the hanger (headstall) hanging close to the outer corners of the eyes.
The hanger should lie about 1/2" from the corner of the eye. Some horses with wide-set eyes need to have the hanger pulled away slightly. In that case, use the cheek string lash to position it.
Tip: This also helps to retain and stabilize the bosal while still allowing the unrestricted lift and release needed for success. It might save you from walking home.
FYI - The use of a fiador was widespread years ago when first training with tougher, less handled horses and riding out alone in vast areas. It was a vital part of the hackamore (that means "halter") and helped to keep the hackamore on the bronc. Many times this was accompanied by the "tapa ojos", a blindfold that pulled down to cover the bronc's eyes. If things got a little too "western," the tapa ojos could be useful. With some horses that is still a good way; but the fiador, by its very nature, interferes with the lift and release o the bosal. With today's horses and methods the hanger with cheek string lash is a workable solution.