often it is easiest to achieve equal tension when you use the same thread, top & bottom- if you use a 'bobbin weight' thread in the bobbin and a heavier thread on top you will have to adjust for the differences...probably loosening the top tension because the heavier thread will be (tighter) going through the tension discs.
i have used 30 wt top thread with a 60wt bobbin thread and not had problems- when my nephew tried a 12 wt thread one time we really had to do some adjusting---he did wind up using a heavier wt in the bobbin (30 i think-but it was a long time ago i may be wrong about that- i know it wasn't the 12 though.) he did finally get it right. but all machines behave differently so all you can do is try & adjust- try different combinations until you find what works...you don't say how heavy of a thread you want to use...but the closer to the same wt as possible will need the fewer adjustments (like using 30 wt top with 40 or 50 wt bobbin would probably work well)