exh open.... depends how fast you can burn fuel, how good and large the port is,, and how high are you going to rpm it
exh closing... usually try to make overlap somewhat symetrical about tdc depending on head characteristics,, rod ratio,, header style, desired idle and driveability
int open... driveability,, size of port respective to displacement,, effeciency of int port velocity
int close... effeciency of port flow/velocity,, length of int runner,, dynamics of manifold to generate inertia,, desired rpm,, mim port cross section
from those 4 numbers you get duration,, then you get sep from there
example,,,,,
i have a decent motor i want to make 950hp with,,, run to 9000 rpm,, high comp,, so ill open exh valve at 85bbdc,,, close at 27atdc,, ill open intake at 32 btdc, its a race motor,, idle at 1500,,, ramp on exh is slower than intake so 50 number is always longer on int since overlap is really looked at 20 numbers,,, int closing at 66,, should peak at 8500 there,, due to port cross section,, length of int runner(strength of inertia),, and cylinder fill properties in int stroke...
so then you say..
exh 85 27 292 at .050 119 center
int 32 66 278 at .050 107 center
(107+119)/2= 113 seperation
thats how "normal" people figure out a cam,, there are more parameteres that can be considered for figuering each event,, but in general,, thats how its done,, so you can see nobody will just say 8 degrees split is good,, if they do they are basing it off a similiar engine,, or have no clue about cams
jeff