buy the dax and selling the stoxx on a 3:1 ratio is not really arbitrage... more correlation based intermarket spread trading. and the Dax is not a very good market to spread unless you can take a lot of heat. The professional local traders do spreading for a tick or two typically against the bund/bobl/shatz in london, but they arent using IB, they will be getting pro clearing rates and using an autospreader like TT X_trader Pro.
Its possible to arb 1 big pit S&P against 5 x emini's for single ticks.. but you rely on having someone in the pit... there are guys who do this full time if you ever get to go the CME floor, they have Playstation game controllers in their hands and use them to buy the electronic markets against the paper on the floor. Or you could look at the after session big contract which is traded on globex vs the emini, but the emini is usuall too thin & bid/ask spread too wide in after hours for there ever to be any significant bid/ask overlap between the two markets. Besides, you'd never see more than a tick overlap and you'd have to do 1 x big contract vs 5 x mini's to get your tick so you'd better be on a very low commission to make anything, and you'd need a faster connection than the guys with TT who are sat in the exchange with a LAN connection to globex.
You can arb between the front & back months & the exchange spread market, but again you're talking single ticks - the calendar spread markets in the stock indices are quoted in smaller ticks, ie: the S&P emini spread is a 0.05 ticks or $2.50 value... bearing in mind you need to buy the front, sell the back and sell the spread = 4 trades in total for a $2.50 tick... you see where the uber-low commissions become a factor to make anything, assuming you actually manage to hit an arb opportunity and dont get legged up.
I've seen it done, but it was being done by pro's in a very slick setup using proprietary software and in house exchange gateways. impossible to do via a retail broker for cost, data speed & execution speed reasons.
By all means look into spread trading, but I think you'll find it very difficult to automate effectively in excel.