I made a spreadsheet for calculating the most efficient increase of damage per SP spent.

https://docs.google.com/spreadsheets...W87lKNi78TxIEo

How to use:
1. Copypaste it to your own spreadsheet so you can edit
2. Fill in "Your stats" as shown in game next to your rune
3. Edit "Upgrade" and "Cost" to the value of the talent you are looking for and the cost
4. Look at "Effect/Cost" to find what would be the best bang for your buck