Whatever works. I'd been thinking in terms of 1mm or so - which will raise the shard up by that much, naturally. If the shard isn't flat it's going to be more fun bending the bezel to fit (but you'd hit that anyway)... Fun, but not impossible.

The last irregular bezel I did I wrapped paper around to get the outline of the stone, added a couple of mm either side then cut that out of .5mm sheet. Soldered the ends, added a bail, then formed it to fit the stone. I used a drop of superglue to hold the bezel while I got the cardinal points of the bezel formed on either side, then carried on as normal. The superglue came off after a dunk in acetone - but I probably didn't really need to.