May I suggest that you get hold of two j bend spokes and cut to 200mm in length. Then screw nipples on so the driver flats are flush with the top of the spoke treads. Place the cut spokes with nipples in the rim at opposite end and pull tight and measure the distance between the ends. Add 400mm and this is the ERD.
Then get a pair of calipers and following the instrutions on this site to measure the hub. http://lenni.info/edd/howtomeasure
Then download this and plug in the measurements and bingo you have your spoke lengths. http://sheldonbrown.com/rinard/spocalc.htm
This what wheel builders do and I think it is good pratice to get used to how to do it your self.
An ultegra rims and DT 250 hub (do you mean a 240?) is not a normal combination so maybe some has done it before maybe not.