In which way is /dɑːtə/ more formal than /deɪtə/?
Solution 1:
Even in formal environments, I hardly ever hear /dɑːtə/ any more. 15 years ago it was, while not common, still in usage in computer halls (back when they were computer halls) but it seemed to vanish along with the move away from mainframes to PC based servers.
Solution 2:
The OED gives only /deɪtə/. I've not heard anyone say /dɑːtə/ and if I did I would think it pretentious.