Ruling and shading

Sorry if I'm being a bit thick, but I can't seem to work out how to have alternate body rows in my table shaded. The dialog has "Shade by.." and I've chosen "Body Row(s)"

It then has a box labelled First - for this I've put "1", as I want body row one to be shaded (and I've then selected Fill: 10% and Colour: ...)

But this is only making row 1 shaded. How do I get it to continue on alternate rows? i.e. what am I supposed to put in the "Next" box?

I've tried "2" and "None" for fill, and "3" and "Fill 10%" but I then get all rows shaded or every 4th row shaded or ...

Any suggestions? The significance of the "First" and "Next" boxes isn't explicitly mentioned in the online help.