Ian, the 105/2.8 AI/AIS MicroNikkor has a floating element that reduces the lens' focal length as the lens is focused closer. At its close focusing limit, according to Bjorn Roslett focal length is 88 mm. So your arithmetic is a little off.

The lens goes to 1:1 on the PN-11 extension tube. The range of magnifications available with the lens on the PN-11 is 1:2 to 1.14:1

You may have been thinking of the 105/4 MicroNikkor, which goes to 1:2 on its own mount and from 1:2 to 1:1 on the PN-11. It does not have a floating element and its focal length does not change as it is focused closer.

Dave, if you want to go much above 1:1, hand-held won't work, even with flash. The problem is that unsteadiness will move the plane of best focus. Shooting my 105/2.8 MicroNikkor AIS hand-held at 0.88:1 isn't easy, and I use flash.

But if you have faith in your steadiness, perhaps you should abandon Nikon and switch to Canon. Canon's 65/2.8 MP-E goes from 1:1 to 5:1 on its own mount, talks to all EOS mount cameras.