After much initial frustration (!) I found a solution to the slashing
problem. First, the cut must be quick and decisive, 2 seconds typically;
second, I used a heavier blade (a butchers knife) that had been wiped with
water. The heaviness seem to take care of making a 1/4 inch deep, even cut
without thinking about it, just drawing the blade along, held loosely in
the hand, no pressure at all. Prior to that I had some success with the
single sided razors you can buy in DIY shops, but with them I had to exert
cutting pressure and got uneven depths.