I'm assuming you're currently leveling up, and by your question's nature I'm guessing your in your mid 20s, early 30s.
In short, stack Intelligence and a bit of Stamina (of the Eagle works great), to smooth out your leveling experience - stamina being especially important on a PVP server and not so much on a PVE server.
When itemizing a character, its important to keep in mind your main goals. On the grind/quest of everyday WoW, small ups in Damage usually will not give the same long term benefits as stacking more of your Mana and Health pools. When you start to get around level 55, and on throughout Outlands and Northrend, you will find yourself having more options than just "Spell OR Intel" and then you choose your stats based on your spec. But when it becomes a sacrafice to your base manapool in order to up your damage a marginal amount, its not worth the trade off and you will see your killing capacity as well as kills per hour fall.
As for a formula, it is difficult to calculate such a formula due to the fact that the two stats do not function on a similar playing field. Spell Power will make you kill faster, where Intelligence will let you kill longer before a break and, being a Mage, will allow you more escape and kiting options that use a lot of mana.
How Blizzard has actually formulated it (with their suffix and iLevel formulas) is the following ::
1 Int = 1.25 SP = 1.5 Sta