I have the Specific Gravity stuff in the shed. Don't use it any more. When all these country wines were developed nobody measured the SG. As long as you have enough sugar to feed the yeast and not so much that your wine is too sweet you're ok. You can always add sugar later if you want to up the alcohol. You can't take it out! I also found from experience that adding all your sugar at the beginning can make the fermentation stick. Doesn't always of course, but it can.
Did A Level chemistry BTW - doesn't really help!

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vegheaven.blogspot.com Updated July 16th 2008