Some latency is inherent when you're using MIDI, so yes it's normal. But 30 feet is nothing when you're travelling at the speed of electricity!

The delay is mostly due to the speed at which your MIDI module triggers. The way to minimize it for real-time playing is simply to use a fast-triggering sound module like the Alesis DM5, which has excellent timing. In fact, it has trigger inputs with faster timing than MIDI; you can still record the MIDI output for tweaking during playback.

I don't know if there's a formula, but you can record the triggered sounds on a separate ADAT track and measure the distance between them and the triggering sounds.