Who told you that ''exposing for the shadows, developing for the highlights.'' is usefully related to film speed in that manner? You tried it once at half rated speed, then decreased dev time by a small amount and got the opposite result. That should suggest to you that your approach is random and based on a false assumption.
Your film speed is based on the way you expose and develop film. You should test specifically for that, with each film, developer, camera, if other's findings don't work for you. That will also give you an idea of how much to decrease or increase developing to contract or expand development. You have to know where you are before you can know where you're going.
How can any of of us know how to help you without your exposure information? Perhaps it is incorrect exposure, by you or your tool or both – based on the scene, and not the 20% less contraction. I would be very surprised, if given what you said is accurate, that the issue is in any way dependent upon developing time/temperature at all.