You can attack this problem from two sides:

 - **Start with the available drawings and specifications:** It helps a great deal to compare similar drawings from a number of different manufacturers; say, for example, 0805 capacitors from AVX, murata, Kemet and whatnot, or MSOP-8 ICs from TI, Fairchild, Linear and whoever. Then, try to create footprints that agree with any of these manufacturers' standards and min/max tolerances. If you have access to sources that are not specific to one single manufacturer, like IPC (Association Connecting Electronics Industries) or JEDEC, and you can bring them to agree with what you've figured out, even better.

 - **Start with information based on practical experience:** Ask your place-and-solder shop what works best for their soldering processes and machines. They will likely know that pads with a slightly larger or smaller dimension in one or another orientation help them to avoid unwanted things like tombstoning of chip capacitors or resistors.

Ideally, all the information you can gater will not contradict and you end up having Your Pefect Decals<sup>TM</sup>.

Related, but not a duplicate: http://electronics.stackexchange.com/questions/13072/the-right-0805-footprint-land-pattern