Joe's right. it's pretty arbitrary. The only reason they had five-digit stardates in TNG and beyond was originally to differentiate them from TOS. But then they figured they should keep track of the stardates and make them more of a linear progression.
I find it easier to think in terms of dates. The stardates for the first season of TNG we 41XXX.X and the year was established (in "The Neutral Zone") as 2364. Gioven that the first two digits go up once a year, subtract 41 from 2364 to find out that the five digit stardate cycle began in 2323.
If your game is set after 2323, subtract that year from the year your campaign is set to get the first two digits. For example, if your game is set in 2378, then it would use stardates begining with 55.
The remaining three digits and the decimal are determined by what part of the year you are in. 1000 "units" per year. You can either fake it or try to do the math to figure it out. I believe there is a Stardate Calculator (Excel Spreadsheet) in the COmputer Core here at Trek-RPG Net if you want to be more specific without having to dso the math yourself... 
Mind you, the above discussion only applies to the five-digit stardates. The four digit dates from TOS don't really follow any pattern (other than they got closer and closer to 9999.9 the further they got in the movies). If you wish to run a TOS game, just fudge the stardates. Everyone else did!
Former Decipher RPG Net Rep
"Doug, at the keyboard, his fingers bleeding" (with thanks to Moriarti)
In D&D3E, Abyssal is not the language of evil vacuum cleaners.