Tokina 100mm macro 1:2 and nikon pk 13

Does anyone know which would be the macro ratio if I mount a nikon extension ring pk-13 (or the pk-11, pk-12) and a tokina 100 mm macro 1:2 on my camera (a nikon f80?

Try holding the lens off the camera by approx 27.5mm or whatever the extension you choose and look through the camera at a ruler, that will show you without having to buy the extenders just to find out. 1:1 would be about 36mm across.
Cardboard tube or an assistant may help!
Calculating is harder.
The magnification will obviously vary with the extension due to focusing using the lens focusing ring.