I'm not sure that it matters if the baffles are sewn at the end. I really can't picture much down shifting through the tiny end gaps. I wouldn't worry about the slight loss of insulation on the edges if this is a summer or 3 season quilt.
You will have to look at a chart and then do some math or use a fill calculator. Here is the generally agreed upon loft vs temperature curve. It's obviously varies if your a warm or cold sleeper. Once you know what your loft is, you need to take and calculate the volume of your quilt in cu. in. and divide that by the fill power of your down to determine how much you need. The fill power equals how many cu. in. one ounce of down will fill. Cheater calculator here.
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