Most players havent found a good way to compare items and stats just yet. Well a spreadsheet has been made that gives you all this info to help you. The creator of this nice spreadsheet made an easy to follow guide for it. Here is the guide plus a link to the spreadsheet
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You need to know the coefficients of your spells to make effective use of it. If you search the forums you might find someone who has already figured them out, but they may have had talents that skewed the numbers. To figure it out for yourself, cast a spell with no talents that increase the damage (+ life, + dmg, etc) on the targeting dummy several times to get the min/max hit. Subtract the average tooltip damage from your average actual hit, and divide that number by your spellpower.spreadsheet