Not enough!!! .... for any performance cam. Tough to measure depth without a reference point. There is a tiny 'v' between the valve reliefs that is about 1/8: high. When I put my last 401 together, I measured valve-piston clearance without any valve lift from the cam. At TDC, there was 0.186" clearance on the intake and 0.196" on the exhaust. This was with Zollner pistons, steel head gasket, and about 0.007" taken off the heads. Pretty close to a stock build. With a Poston NH400 cam, v-p clearance was 0.097" on the intake with stock rockers, 0.118" on the exhaust. With Toms roller rockers, clearance decreased to 0.072" intake, .092" exhaust. The NH400 cam has 114* lobe spacing. A cam with tighter spacing would have had even less vp clearance. Next time I build a NH, I'm going to notch the pistons to fit a decent cam. Haven't decided on the best method to do so.... The Isky cutter is nice but expensive.
You can zap a brazed lathe cutter to an old valve and use the head as a jig if need be. Best if it has a radius. A stop collar helps with the depth setting.
I bought cutters from "Edge Tools". Guy makes them any size you like. Reasonably priced, bad ass cutters! Dave
http://www.etsupply.com/edgetools/lindyfiles/inheadpiston.html Lindy tool has some decent cutters too: http://www.lindytools.com/ihpc I just bought a pair of Lindy's from eBay. They're spec'd for a LS1 so I'll need to resize them for my Nails.
If you end/end up using the pistons I specd you won't have to worry about notches or LSA as they are MADE to accommodate up to .700" lift.
Unfortunately other piston manufacturers didn't do the planning Tom did with his pistons! My first 425 build used forged TRW's. Even those needed valve notches cut to accommodate the Isky 310 cam, and it has under 0.460" lift. It isn't the max lift that causes v-p clearance problems, it's the open/close points of the valves. Directly related to the LSA as Tom mentioned, and the duration. VP clearance is tightest during overlap as the exhaust is closing and the intake is opening.... about 10-15 degrees before and after TDC
I just saw this today, had to share as I thought it was fitting. Now back to your regularly scheduled programming!