The drop out rate is based on Today's Queue Number minus the total units taken, den u get a rough percentage.
U can get the today's q number info from MyHDB.
My HDBPage >
My Flat >
Application Status - New Flat >
Click on ur Ref No.
It will be shown as follows.
Last Queue Number invited for selection of a 4-Room flat in Kallang / Whampoa under the May 2012 BTO exercise as at 20 Jul 2012 : 00070
For example as of today's date.
Total Units available: 888
Units still available for selection: 840
Units taken: 888 - 840 = 48
No. of q drop out: 22
Queue number as of today: 70
Drop out rate: 22/70 x 100 = 31.4%
Btw, u may wish to check out HWZ May 2012 BTO forum.
There might be more helpful information available.
Hope this helps