Unfortunately, you're the latter: waaaay off.
The reasons?
1. The math for clearance is a fuzzy mess. You have to include all sorts of measurements when you wanna find out how far the piston actually is from the valve (head gasket thickness, piston height, etc) and
2. The valve TIMING is more important to determine exactly if and/or when the valves are going to hit the piston.....well, hopefully they don't, but if you're in for some flycutting, this is your bag. This is why a lot of guys wonder what the maximum lift you can put in a stock 5.0 is, and there really isn't an answer because every cam has differently timed events. Think of it this way. When your exhaust (or intake) valve is full-open, is it right at TDC, a little after, or a little before? This is where it gets fuzzy. Oh well, hope I didn't confuse or discourage ya. Later!