This sharpening guide, bought in large DIY area finally allowed me to get really good results, reproducible.
The dual face of this kit stone to alumina grains, do not agree to addicts purists of Japanese stones (!), But that gives more correct results.
The accuracy of the folding wedge is relative, because the 25 ° graduation actually corresponds to 22.5 °, but that did not matter: just always lock on the same scale or spot once for all the graduation on the block corresponding to the desired angle.
Next step: I will test this kit on a jammed belt sander upside down in a vise for quick and effective result.
To be continued ...