Hi,
You should select metric units to calculate the areas properly as you correctly suggested. Then after automatically calculating the areas in "ha" or in "m^2", you change all the area values to english units means either in "ac" or in "ft^2" by calculating yourself. Then change the unit option to english units and re-enter all the converted values in the respective area fields and continue and finish your project in english units only.
Thanks,
Debajit Basu Ray
debajit@bossintl.com