My first question would be a less technical one, why are you attempting to constrict yourself to 250mb, with the cost of traditonal magnetic hard drives these days, even the smaller drives would leave you with plenty of space for very little cost.
You can download formula to work out the closest estimate if you really want to be that precise. Industry advice is to always add a 1/3. you should also be sizing your database & log so they do not expand quickly, use a sensible initial size so the database does not expand in the near future. Logs will also be used for reindexing which will give you spikes in the log capacity needed.
basically make yuor best guestimate, add a third, leave sensible increments for database expansion and leave autoexpansion on, DO NOT USE autoshrink. Monitor the increases in size with the built in SQL 2008 reports..