Many people believe that managers should step back and concentrate entirely on strategy and management. It makes sense that managers are expected to spend the majority of their time on these things. But as an industry, we pay too high a price for our managers when we allow or demand that they barely spend time coding. Once a person stops coding for a significant portion of time, critical connections to the concerns of developers atrophy. When that happens, decision-making, planning, and leadership suffer, undermining the entire basis for promoting engineers to management positions.
I'm not sold on the 30% thing, but an engineering manager should develop software at least some of his or hers time. Fix a bug, write some unit tests or help develop a small feature. Be able to build your product and understand its architecture.
Don't loose your engineering mindset!