Basically, this is a measure of how much of your business activities occurred in this particular Home Office.
If you earn all your self-employment income using this Home Office, enter 100%.
If, however, you earn part of your income outside the home (cold calls, another office, sales meetings, etc.), enter only the percentage that is earned in your Home Office.
If you had more than one home office during the year, enter the percentage of the income that was earned while using this office. The balance would be reported for the other Home Office.