Sorry, you're dead right, I misread the calculator, 3 kg should only give 7.1 (so the Coopers yeast wouldn't have been fazed !!)
If anyone's interested, here's the code, just copy it and insert into Notepad then save as alcstrength.html or whatever you feel like - as long as it's an .html and you can open it with Firefox or IE and it should run fine:
<html>
<center>
<h2><font color="#ff0000">Preparing a Sugar Based Wort</h2></font>
<form name="sc">
<center>
<table bgcolor="#add8e6" cellpadding="4" border="2" rules="none" bordercolor="black">
<tbody>
<tr>
<td colspan="3"><input size="4" value=" 5" name="a">
<select name="weight">
<option value="1">kg</option>
<option value="0.45359237">lb</option>
</select> sugar made up to
<input size="4" value=" 20" name="b">
<select name="amount">
<option value="1">L</option>
<option value="0.946352946">US qt</option>
<option value="3.7854118">US gal</option>
</select> total volume
</td>
</tr>
<tr>
<td colspan="3">should have an SG <input size="5" name="s"> and only require
<input size="5" name="wreq">
<select name="wreqt">
<option value="1">L</option>
<option value="1.05668820943">US qt</option>
<option value="0.264172052358">US gal</option>
</select> of water
</td>
</tr>
<tr>
<td> and should produce a wort of <input size="5" name="at"> % alcohol
</td>
<td align="right" valign="bottom"> <input onclick="
x=sc.a.value*sc.weight.value;
y=sc.b.value*sc.amount.value;
sc.s.value=Math.round(((258.6+(87.96*x/y))+Math.sqrt(66874+(7736.96*x*x/(y*y))+(57947*x/y)))/517.2 *1000)/1000;
sc.wreq.value= Math.round(((y * sc.s.value) - x)*sc.wreqt.value*100)/100;
sc.at.value=Math.round(sc.a.value*sc.weight.value*1000/17/(sc.b.value*sc.amount.value)*10)/10;
"value="Calculate" tr="" type="button">
</td>
</tr>
</tbody></table></center>
</form>
</html>
