the great ting about a scarf / shawl is that it does not so much depend on gauge. It does not really have to fit. Make it the width you want (you can try that real quick on a few rows) and keep on until you like the length.
If you feel like needing more yarn in the end, there is good reason and good excuse to buy some. Then you also won't pile a big stack infront of your hubby. He will only see some extra and a big project to admire!
after doing the pattern repeat (rows) once, you can measure and find out how long things will get if you repeat as often as you can make it.
Usage of yarn you can find out real easy: weigh your needles before you start (if you are not working with seperate ones (those you can weigh later)). Then do a part of your pattern. Measure how long, e.g. 10 inches. Then weigh your piece, deduct the needle and you know how much yarn you used for 10 inches. Then you can do the math.