My daily range is similar, maybe a bit less. 30-50km and a few days a week with short 10km or no trips.
I charge to full, then I don't recharge it until I know my range is getting close to my next days trip. If I'm not sure of my trip tomorrow and my battery level is under 50%, I charge it anyway so I don't have a concern about range.
Charging to 70% would be reasonable for my use, but there is no automated method for this on the car or charger. It would mean more frequent charging for my use though. BMW makes no mention of it in the manual, but they reserve some battery capacity to protect the battery from overcharge or depletion. I think if it was important for the i3 battery, they would have made these allowances, they haven't.
To make it easy to charge to 70%, you'd need charger control. I think the top model Juicebox would be the only one with this capability. You'd have to work out how many KWh it will take to bring the i3 up to 70% and then dial that into the Juicebox. No way I'm getting up at silly hours in the morning to turn off the charger at 70%!
Unless BMW make provisions in the software to limit charging to 70% capacity I can't see a reasonable user-friendly way of implementing it, so even though it appears to be a reasonable idea I'm passing on that one.
2014 BMW i3 BEV deposit placed 19th Dec 2013
Delivered in Melbourne 4/12/2014. Arrived locally 11/12/14, with 24km on the clock.