hi nigel,
i have always followed what i call the SMEE 'standard' as expounded by the late Bill Carter.
instead of equal bar width and air space, use say 3/32" bars with 1/8" gaps, 1/8" bars with 5/32" gaps, and 5/32" bars and 3/16" gaps. for 3mm stainless strip i would use 5/32" gaps obviously.
with the deterioration in steam coal over the last 30 years i think wider gaps are more important than ever, though i have a stock of very good welsh steam coal in a shed that will outlast me!
cheers,
julian