Sprint planning is typically 4 hours [1]. I think there are further reasonable quibbles to be made that would revise that 7% number up to a reasonable 13% or 18%.
The 13% takes into account the time tax of context switching. 18% if we consider only (at best) 6 hours per day and meetings are taking away from that high productivity time (which is to say a greater proportion of time is spent around the coffee maker and not coding)
To go into detail, there is a tax to every meeting. With 13 scrum meetings this adds up fast. For me, as a remote worker, I stop everything 5 minutes before or else (for example) 1:57 is going to suddenly turn into 2:03 and then I'm late. Recovering from a meeting takes me about 10 minutes (plenty of resources state it takes 25 minutes to get back into a flow state once interrupted).
Thus, for me, the meeting tax of scrum meetings alone is 13*15 = 195 minutes. Further, there is a conflict as projects have their own required meetings (if the scrum meetings are the only meetings, then seemingly the team is just talking once a day and then not again - that sounds bad! So yeah, there are other meetings too).
Further, not all 80 hours are equal. Software development is a highly creative field, it's not an assembly line with interchangeable parts. If you have your developers in office for 90 hours instead of 80 you're not going to get 10% more on your story point velocity. Similar for 70 hours vs 80, eventually there is a point of diminishing returns.
By measures of how long I can pair program with someone, 5 hours is a pretty upper limit on how long I can stay in a high productivity state for one day. Time spent in meetings can take away from that high productivity.
Let's say a person can generously be in a high productivity state for 6 hours per day and that meetings take away from that time, then we are very reasonably at 10.75/60 => ~18%*
*I'll emphasizes we are talking scrum meetings only, and still not even taking into account prep time for the scrum meetings
"The general rule of thumb is to allow two hours of sprint planning for every one week of sprint length. That means teams should timebox sprint planning to four hours for a two-week sprint and eight hours for a one-month sprint."
The 13% takes into account the time tax of context switching. 18% if we consider only (at best) 6 hours per day and meetings are taking away from that high productivity time (which is to say a greater proportion of time is spent around the coffee maker and not coding)
To go into detail, there is a tax to every meeting. With 13 scrum meetings this adds up fast. For me, as a remote worker, I stop everything 5 minutes before or else (for example) 1:57 is going to suddenly turn into 2:03 and then I'm late. Recovering from a meeting takes me about 10 minutes (plenty of resources state it takes 25 minutes to get back into a flow state once interrupted).
Thus, for me, the meeting tax of scrum meetings alone is 13*15 = 195 minutes. Further, there is a conflict as projects have their own required meetings (if the scrum meetings are the only meetings, then seemingly the team is just talking once a day and then not again - that sounds bad! So yeah, there are other meetings too).
Further, not all 80 hours are equal. Software development is a highly creative field, it's not an assembly line with interchangeable parts. If you have your developers in office for 90 hours instead of 80 you're not going to get 10% more on your story point velocity. Similar for 70 hours vs 80, eventually there is a point of diminishing returns.
By measures of how long I can pair program with someone, 5 hours is a pretty upper limit on how long I can stay in a high productivity state for one day. Time spent in meetings can take away from that high productivity.
Let's say a person can generously be in a high productivity state for 6 hours per day and that meetings take away from that time, then we are very reasonably at 10.75/60 => ~18%*
*I'll emphasizes we are talking scrum meetings only, and still not even taking into account prep time for the scrum meetings
[1] https://resources.scrumalliance.org/Article/scrum-events
"The general rule of thumb is to allow two hours of sprint planning for every one week of sprint length. That means teams should timebox sprint planning to four hours for a two-week sprint and eight hours for a one-month sprint."