The distances are measured on-axis.
means that the diaphragm goes after the second surface. The lenses are 1.3 mm thick, the distance between them is 12.6 mm. You got it right.
If you click on "courbes" you'll see some calculated performance data. The news isn't good, horrible curvature of field. But since the OP wants to use a long lens on a small format, i.e., need the lens to cover only a small angle, he may be able to get away with it.
The calculated spot diagrams are dreadful too. The world gave up on periskop lenses for good reason.