How do you cover a cake in fondant triangles so that it is even.....how do you know what size to use? Is there a formula or something?

I'm not sure what you mean by triangles (do you mean on the top, on the sides,square or round?). I used a template for my pleated fondant cake. I made the template from computer paper. In that cake, the only math would be for bottom width and since I had a back panel I didn't have to be too exact.

sorry, I mean like your Christening cake the bottom tier. Thanks

Summersusu,
I use the Ateco diamond cutters which is a set of several sizes. It's actually quite difficult to get them to line up perfectly. I guess you could do the math but I choose the size based on looks and then when you get to the back you kind of adjust the spacing. You may leave more space between the ones in the back or have the last couple overlap. The real trick is to pick the front of the cake and apply several diamonds to the right then go back and apply several to the left so that you're working your way around the cake on both sides.

I actually did a write up on that cake at http://www.keeponcaking.com/2011/03/christening-cake/

Also here on CC I saw someone who started her diamonds and when she got to the pint where they wouldn't line up she left a large white area and filled it in with a monogram. So in her case the 'back' of the cake was used as the front. It looked really great.

I hope that helps.