Bart the belgian using a flash, it's not necessary to go so high in ISO. F/13 isn't to closed according to me but it depends on the lens. You may adjust flash power to get much details. Personally I'm using always the same parameters ISO 100, F/11 (best compromise between DOF and sharpness with my lens) and 1/250s to be able to freeze my subject and avoid hand shaking. I only change one parameter regarding the magnification and this is the flash power. Only exception is when I want to do focus stacking and then I use ISO 200 to be able to lower flash power to 1/16th to shoot in burst.
Thanks. I think that Pringles tube is, as discussed offline with Helix_2648, an easy way to bring the light where we want to. But (yes there is a but) the diffuser on this system still too much small according to me. If I was using this system, I'll cut a bit the tube to be able to place at the extremity a small softbox.