WCB pays 90% of net based on your pre-accident earnings.
They calculate it using your annual gross pre-accident income going back from the date of your accident. From there they figure out 90% of net and issue benefits. WCB will deduct whatever your post accident earnings are from your benefits. EG: after you got hurt, your work gives you light duties at 20 hours per week, but you were working 40 hrs per week before. WCB will issue the difference.
But, there is a catch. WCB only insures a maximum of $86,700 per year. If your gross earnings before this is greater, only the first $86.700 is insured. If you don't earn this much year it won't affect you.
You do not pay income tax on your WCB benefits
Go here for more info:
http://www.wcb.ab.ca/workers/wage_replacement.asp
Edit: Forgot to add: If you haven't worked for your current employer for a year, you can also use previous income (within reason and no gaps between employment). You can also include earnings from a second job if you have one right now.