CS: Point taken and I agree, blue line marked on the ASI is for a certain set of conditions, and the speed which will give max SE performance changes when conditions change. I guess what I really was asking is: What relevance to the departure you are about to conduct, has a speed relating to a different set of conditions? "Blue line" on the ASI has no real relevance, what you want to know is "blue line" for this particlular T/O. This goes back to K.I.S.S. - and that's another good point too.
Decision points, as far as I was taught anyway, only relate to light twins off loooong runways, where you are able to reach a flying airspeed with still enough length to stop on the remaining runway. Gear up when you have insufficient length remaining, that is your decision point. By then you (should) have the airspeed required to proceed - in these cases I select gear up, and then straight to the ASI to confirm this. To me, it's not so much about your decision being made to GO, AFTER gear up - but to ABORT, BEFORE gear up.
A previous chief pilot told me once: "What's really important is not so much what you do, but whether you can justify your actions afterwards"!!