Duration Not Converting Correctly from Hrs to Days-Calendar and Admin issue

Hi

i'm having an issue with P6.7 where i have created a 7 day 24hr calendar for an activity. when i view the duration in hours it is correct but if i convert it to days it calculates it back to the admin timeperiod preference of 8 hr day. so basically a 24hr activity displays a 24hr duration but when i convert to days it times it by 3 to give it a 3 day duration. i've checked the box 'use assigned calendar to specify the number of work hours for each time period". i would have thought this would have overridden the admin preference setting?

any advice is most welcome

thanks in advance

Mark