Additional damage is based on the total number of dissolved runestones. Each of the first 5 runestones increases the attack by 0.15, each of the following 5 runestones increases attack by 0.13. From the 36th runestones onwards, the attack will be increased by 0.01 per dissolved runestone.
3 runestones dissolved = x2.25
6 runestones dissolved = x2.68
9 runestones dissolved = x3.07
12 runestones dissolved = x3.42
15 runestones dissolved = x3.75
18 runestones dissolved = x4.02