All refinement levels have a range of percentage of a stat that you will gain at each level of refinement. A +10, for example, will have a greater stat gain range than a +1. While the stat gain you receive from refinement is random, you can increase the number of materials you’re using in order to increase the odds of gaining the maximum effect for that enhancement level. If you’re unhappy with how your refinement went and want to try to get a better result for that particular level you have the ability to reroll that level to try for a higher stat gain. In order to refine your equipment, you will need to acquire the material which is listed on the refinement screen. Simply use the required amount of materials that were used in the attempt. You are able to increase the number of materials which can be used on an attempt by pressing the + button. Reaching 100% guarantees you a successful refinement attempt, with anything above that increasing the chances of a higher stat gain for that refinement level.