Hi John,
You could try to get a copy of British Railways Standard Steam Locomotives by E.Talbot which has a copy of the BR lining drawings and painting instructions for all BR liveries, Blue, Green and Black mixed traffic.
Mixed traffic lining as follows:
Boiler bands 1/4" red line each edge. Cylinders similar pairs each end.
Footplate edging from the bottom edge 5/8" grey – 1/8" cream – 1"black – 1/4" red – black to top.
Cab and tank sides working in towards the centre 5/8" grey – 1/8" cream – 1 5/8" black – 1/4" red. No indication of the distance the lining starts fron the cab or tank edge but I suspect this varied depending on the loco type.
The outside radius of the corner curves is 4".
Hope that's helpful.
Eddie