ES 3" field flattener back-focus measuring point

[Bikeman55]

The title pretty much says it all.

Where do I go about measuring the 55mm of back-focus on the Explore Scientific 3" field flattener/reducer.

I have attached a couple pictures for reference so we can refer to the same locations.

I thought from the research that I have done I should measure from point A but after last night's first use it seems as though I got something wrong and I wanted to make sure I was measuring from the correct spot before I really start to scrutinize my spacing to sub-millimeter or perhaps my imaging train is not quite "plumb".

Any insight would be greatly appreciated.

Thanks.