It doesn't exist, the trick is knowing which country's sizing you are looking at, the shoes can be marked for the size in the country they were made in, or intended for export to. I have found that the majority of pre, 1960's british shoes used american sizing. That isn't exclusive, there is the odd company that used English sizing but it seems to be far rarer.
You can start guessing the country's sizing from the way it's written:
UK can appear: 6 or 6 1/2 or even 60 (size 6) on older shoes. width letters on british shoes are less common and if present go from C, D is standard width in Clarks at least, has been for some time. E is wide, F extra wide. they tend to be marked slightly apart from the shoe size, unlike US methods:
US tends to go 6 or 6.5 and with width letters directly after the size number: AAAA, AAA, AA, A, B Narrow to standard in varying degrees I believe.
Italian sizing is one size big, so always look out for 'made in italy' and double check.
US sizes are traditionally one and a half sizes above UK, I was told by Rayne this was the difference on their sizing chart. However more modern shoes tend to be closer to 2 sizes difference between UK & US.
If in doubt, measure carefully inside with a soft tape measure along the insole and compare against a pair you definetly know the size of. Remember high heels will have a longer insole measurement than a flat shoe of the same foot size so you need to account for this.
P.S. I have used that chart before and would say you cannot rely on it as the measurements of shoes in my size do not match the sizes given, especially the crossover's seem to get more far fetched towards the larger end of the size chart. I would probably refer to it for an unusual country's sizing only.