Problems with the lichenometry

The picture below shows many small lichens growing on the same stone. I didn't measure them but based on the previous growth rate, they will be a decade or two old. This are starting to merge.

The large lichen below has definitely grown round a smaller one of a different species.

The problem with my method is that I might be measuring a merger of different lichens so that they will look bigger than a single one would be. I have tried to get round this by choosing large lichens that have the same shape as the smaller ones.