How much royal icing to cover a 6 inch cake?

[CinderToffee]

I've got a box of Royal Icing Sugar (the kind with the dried egg white in it) - the recipe on the back suggests 4tbsp water and 500g sugar to make enough to cover an 8 inch cake. Does anyone have any idea how to scale this back for a 6 inch cake? It seems a waste to make far too much and bin it, especially as I suspect they are massively overestimating how much is needed. Would half the amount be enough?

I would make the lot and have lovely thick icing (my favourite bit of the cake). I do only use royal icing sugar once a year to ice Christmas cakes and wouldn't use the dregs of a box that had been open for 12 months so there wouldn't be any point saving any.

I made a 9" square cake and cut it roughly in half to give a plain piece to have with Wendsleydale.

The remaining piece (about 9"x5") I used a 2 egg white and 1lb of icing sugar which covered it in a nice thick layer (but I'd made some marzipan that was a bit uneven )

Why not do half , do a thin layer and see how you go, then if you need more you can make a secind batch. It'll be smoother and dry quicker to do two layers.

If the cake is aprox 3" deep, I get that the surface area of the side and top of the smaller cake is 27(PI), and the larger cake 40(PI). So I'd do 3/4 of the amount.
Can someone check my maths? Kim? Zing??