Electric clock calibration

On my 450SLC electric clock, mounted center-dash (between two ventilation outlets), there is a small speed calibration screw on the back, labeled + in one direction, - in the other. Moving this screw does not seem to speed up my slow-running clock, although I have been cautious, moving the screw only a fraction of a turn. Has anyone improved their clock's accuracy by adjusting this screw? If so, how far did you have to turn it? My clock is losing about 6 minutes/day. I assume that to speed up my clock I should be turning the screw in the + direction. Any advice from someone who has done this would be appreciated.