I currently play a Martin D12-28 and played it initially at concert - light strings at normal tuning.

Recently, I've taken to tuning it two semi tones down. like you're doing - the sound is sublime.

I've even considered taking it a further two steps down as Dan has done - I think perhaps I'd need to pick medium strings though.

Whatever you choose, I hope you have as much fun as the rest of us are having with 12 strings.

Take care and have fun - if it gets too difficult, stick a Shubb 12 string capo on the second fret and play whatever everyone else is playing!!!