The best thing to do is just fly them until you are near 3.8v per cell.
If you can't do that what's wrong with the discharge function of your charger? Don't have a discharge function? I would think you could buy an older NiCad/NiMH charger/discharger fairly cheap. You may not hit the exact per cell voltage you want, but it should be close.
Right now I'm taking some of my packs down to 3.4v per cell since it may be weeks before I get a relatively nice day to fly. I'm monitoring the packs and will stop the discharge by hand, but if I get distracted the discharger will shut off at 3.2v for a 10 cell pack and 3.25v for a 4 cell pack that are currently being drained.
Time to go check the packs.