I have these rockers and .015-.020 is what was recommended to me and it seems fine. With these rockers, there's quite a bit of clearance between the hole in the holddown block and the fastener, which makes it a hassle to hold the clearances when installing them. So what I do is use some horse shoe shaped steel shims and put one of these on the shaft for each pair of rockers as I'm tightening the shafts down. Seems to work ok.

shim