Thanks to all.

It actually seems to be simpler than I thought. Using the simplistic aperture diameter = focal length divided by stop rule means that I can work it back from what I have to what I want. The figures are the same as in Ian's table.

Just to satisfy myself, I drew it up. First image is logarithmic, second is linear.

Basically, if I place my new f8 about half way between the existing f8 and f11 it will be about right. Certainly more accurate than I can meter!