PWK Controller described a probable loss of separation event during an opposite direction operation; noting waiting until traffic landed would have been a better decision.

Narrative

Runway configuration was landing Runway 16 and departing Runway 34. The weather was Overcast 400 FT and the visibility was 1 3/4 mile. I was going on Local Control. We called North Satellite for the release and they gave it to us when a ASTR was on a 9 mile final to Runway 16 and a H25B was cleared for take off on a right turn to 040; he was told to no delay to ensure separation with the opposite direction jet. When he rotated and was airborne; I told him to turn as soon as he could and report established on the 040 heading; to ensure degrees divergence. The Collision Alarm never sounded. I was unable to tell if separation was lost. I estimate that separation was 2.5 miles or greater. By the time the H25B reported established on the 040 heading it was to close to tell at that time. I don't think separation was lost. Landing 16 and departing 34 is a normal configuration at PWK during bad weather conditions. The weather was below circle to 34 minimums; the only way with ceiling 400 FT overcast and 1 3/4 mile visibility is to land 16 and depart 34 which is a normal practice at PWK. Recommendation; next time I will just wait for traffic to land on Runway 16 before launching someone off Runway 34 opposite direction; even though it could cause a delay.