There's a nice page or two in the Zeus precision tables book on how to work it out, based on material thickness, radius & angle of bend. I would paste a copy but that would probably infringe the publishers copyright.Rougly how much allowance should I factor in to bending 0.9mm stainless sheet ?
When I did precision sheet metal work, I never used workshop formula, I always found better accuracy (+ or - 5 thou, sometimes less) using method I described above.