How much hash power to buy?

Let say I want to buy hash power to solo mine Ravencoin (Kawpow). How can I calculate how much I would need to buy over a 24 hour period to on average hit a block?

I know network hash rate and difficulty are in the equation, but can't find the information to calculate it.

For example, from playing with 2Miners SOLO calculator, I can see I would need somewhere around 10GH/s. But I don't want to play around till I get the right number. How do I calculate that number which is around 10GH/s?

Do you know how to calculate the required hash power?