Barry,a photographer I worked with for quite a while had a reading prescription bifocal made on his normal glasses and after some time using them,he had the same prescription placed at the upper part of his lens as he constantly had a stiff neck from tilting his head back to use the bifocal in it's normal position on his glasses lenses.Don't know if it would cause you the same problem but just thought I'd toss it out in case it might be of help.