The 'physical stop' is in the VTC. The only way to work out the maximum cam advance you can run is to either ask the cam manufacturer or measure it yourself. If you have aftermarket pistons then you need to measure it yourself unless your 100% sure the piston has deeper valve cutouts than stock (don't trust them - measure the valve cutout yourself). Determining the maximum allowable cam angle involves assembling the cylinder head on the engine with clay on the pistons, pinning VTEC on and fixing the cam angle, then rotating the engine, disassembling and then measuring the clay to work out the piston / valve clearance. Repeat for each cam angle. Valve to valve is much easier as you just need the cylinder head, a very short timing chain, and then rotate and use a feeler guage between the valves.
At the end of this you get clearances at different cam angles, and then you work out what minimum clearance you are happy with, and then add a spacer inside the VTC to limit the cam advance.
As you can see, this is a fair bit of work. The stock Type S and Type R cams have lots of clearance and can run to 50 degrees (maximum cam advance). The Toda N2 cams can go to maximum. The IPS-K2 can run maximum cam advance also, and for the IPS-ME you can run 50 degrees as long as your valve cutouts are a deeper than stock, otherwise around 40 degrees. I think the crower stg 3s have a maximum of around 35 degrees, but don't quote me on that and check with Crower first.
IPS gives the clearance for the K2
here
