When you take this concept of effective working time to the statistician who sees only the time you spent actually at the keyboard and tells the MBA that your effective time was 2-4 hours out of an 8 hour shift, and that MBA wants you to increase this effectiveness by getting it to 100% of the time that he is paying you.. then thats not going to work.
I worked from home for 7 years, and during the brief summers I took 1-2h biking breaks during the day when I was stuck and felt like my mind was grinding to a halt. Often enough I would get unstuck, or at least have lots of new ideas.
On the flipside, I also learned that one of the most valuable pieces of equipment in these circumstances is a lightweight dictating machine. Or a phone capable of recording on a keypress. The ideas would come and go, and if I didn't record them when they popped up, I would spend quite a lot of time trying to find them again.
Taking notes is underrated.
There's a fascinating corollary on "effective working time" research. I've seen the numbers multiple times in Finnish press, but have hard time even finding them now - and can't recall ever seeing these figures in English media.
According to work wellbeing research done (at least partially) in Finland, knowledge workers can achieve approximately 5.5h of effective work a day. Any hours much beyond that are mostly wasted due to the level of concentration required.
This number chimes well with your estimate: 25-30 hours of effective work per week. I'm quite confident these figures would come up again and again.