How to Measure?

A recent Australian woodworking magazine carried an article on "Routing Groves & Trenches" using a jig with a folding fence.

As mentioned by the author, the key to making the jig is to size the said fence "....to match the distance from the edge of the baseplate to the centre of the bit".

Any suggestion how to measure this distance accurately bearing in mind that "...the baseplate may not be centred on the bit..." ?

Thanks yvan