For starters, dont go by the gravid spot, its very misleading especially on platys, mollies and swordtails.. to many people think the gravid spot is a good indication and it really isnt...
The best way to know when they are ready to drop is look for the signs...
Dwelling on bottom.
hiding near plants or in them..
Hovering near heater..
hovering near internal filters..
dwelling near top of tank..
Being very un social..
When the fish is big in the belly and acting like the above, you know they are ready