Base curve values have similarity like our clothes size. Thus, wearing a base curve of 8.6 in one brand of contact lenses do not mean it will be the same in another. Different materials of the contact lenses do affect the base curve as well.So, the best option is to get the trial contact lens to try it own personally before purchasing. Generally, the contact lens that slides downwards while wearing indicate the base curve is too loose. Then, you should try the steeper base curve. E.g if you are using BC 9.0 (flatter) but sliding downwards in your eye, then change to BC 8.6 (steeper).A good fitting of soft contact lens wear in the eyes should have movements of ~1-1.5mm each time you blink. To promote tears exchange and oxygen transmission. Thus, if you wear BC 8.4 lenses that shows no movement in the eye, then you ought to change to BC 8.8. You can seek your contact lens practitioner advise on the fitting. :]