your best bet is to get GSF250 workshop manual. I expect that there will be detailed description of how your carburetor works. It is always good to know how it works if you want to try to modify it.
For needle clip I expect that stock setting is middle position (in case there is odd amount of grooves available) or one the one closer to the near of needle (if there is even amount of grooves available).
Pilot air screw is usually turned 1+1/4 to 1+1/2 turns out from complete closed position as a default but this will provide you only safe starting position for further experiments (in terms: at least it should crank up on first try).
Note that it is always worth of cleaning the carbs thoroughly before trying to set them up because if you let some dirt to stay in carbs it will become loose sometime later and will most probably clog some passage sooner or later forcing you to clean them anyway.
Good luck and hope it won't take long before it will be running again.