pieceofpisces says
Generally you want to be able to sell at wholesale (50% is standard) and still net a profit, if not you may need to look at either suppily costs or price increases (In my case that means 60 gallon oil drums direct from oil presses).
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But keep in mind the voucher % has little to do with what your actual % will be depending on what your prices are. You can offer a 50% voucher but that might end up meaning 40% off or 70% off depending on your item price points, fees, shipping costs etc.
This is an equation you could plug your #s into if you want:
http://www.etsy.com/teams/8380/heartsy-help/discuss/6850168/page/1/I found it easier to do a spreadsheet though!