Ratio math per period
Each percentage is a ratio: a date's position in a period divided by that period's length in days. Position counts day of year, day within the quarter, day of month, or day of week. Lengths are the calendar lengths, which differ: 365 or 366 days in the year, 90 to 92 in a quarter, 28 to 31 in a month, always 7 in a week.