At the end of the exhaust stroke and the beginning of the intake stroke both the intake and exhaust valves are open at the same time. This period of time (in degrees) is know as the Overlap Period. The overlap period has a profound influence on the running characteristics of the engine. It effects both the low speed and high speed characteristics the engine.

There are a couple different ways to calculate the overlap of the cam.

1) The easiest way is to add the intake opening times to the exhaust closing time. Seat to seat times (advertised duration) must be used. .050" durations or timings will not give the correct overlap.

Example:
Comp Cams EX268H
Adv dur: int 268º, exh 280º
110 LSA, 106 ILC (4º advance)
Valve timing @ .006"
intake opens @ 28º BTDC
Intake closes @ 60º ABDC
Exhaust opens @ 76º BBDC
Exhaust closes @ 26º ATDC

In this example the overlap is 54º (28º+26º)
 

2) If the seat to seat opening and closing times are not known, the overlap can be calculated using the advertised duration and the Lobe Separation Angle (LSA). This formula works for both single and dual pattern cams.

Add the intake and exhaust adv durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2

Using the example cam above:
268+280=548
548/4=137
137-110=27
27*2=54
The overlap is 54º
 

Here's another example:
Comp Cams XE294H
Adv Dur: int 294º, exh 306º
110 LSA, 106 ILC (4º advance)
Valve timing @ .006"
Int opens @ 41º BTDC
Int closes @ 73º ABDC
Exh opens @ 87º BBDC
Exh closes @ 39º ATDC

Adding the int open and exh close times (41+39) the overlap is 80º

Using the formula above:
294+306=600
600/4=150
150-110=40
40*2=80
The overlap is 80º
 

These are the valve timing overlap ranges that are most likely to work correctly (thanks to David Vizard for providing this):
                       trucks/good mileage towing 10-35 degs overlap
                       daily driven low rpm performance 30-55degs overlap
                       hot street performance 50-75 degs overlap
                       bracket/oval track racing 70-95degs overlap
                       dragster/comp eliminator engines 90-115 degs overlap

As you can see the XE268H cam falls at the top of the daily driver range and the bottom of the hot street range. It will have a civil but noticeable idle with good vacuum. The XE294H cam is in the middle of the bracket/oval track range. It has a radical idle and low vacuum.