Hi Fireball,
There seems to be something wrong with the equations giving the variation margin on your web page. The actual calculation is OK, it is just the web page that gives the wrong expression.
The variation margin should be the price difference times the size of the futures contract. As the actual contract is a futures contract for -10 USD, traded in BTC, but the price is listed as the price in USD/BTC, a reciprocal comes in, confusing matters a bit, and the equation becomes (on your web page):
Vm = (1/p2 - 1/p1) * W/R
Here 1/p is the "real" price (in BTC/USD), and W/R should be the contract size, i.e -10 USD, giving a variation margin in BTC. However, on the web pages
https://icbit.se/BUZ2 and
https://icbit.se/BUH3 you list W = 0.0001 USD and R = -0.001 USD, giving W/R = 0.1 (no unit). So both the magnitude and the unit is wrong. The example at the bottom of the page correctly uses W/R = -10 (but leaves out all units). And the actual variations margins calculated in your trading platform are correct as well, fortunately
I have not looked closer into the Gold and Oil options, but since the prices here are listed in BTC I am a bit surprised to the reciprocal of the price in the equations.