well i dont breed turtles or anythign but a book i have says that if the temp is 71 F the group will hatch as mostly males and if its at 79 F it will be a fair mix and any where above that temp you will get more females. if you want more females than males hte book suggests a temp of 88F you should check wiht other sources thoughthis is a book for bx turtles, i dont really think the temps would be the same for many other types of turtles...
hope i helped some..
ps. when breeding, if you want to incubate the eggs your self be very careful and plae hte egs in your own substrate that is kept moist but not wet, htey have ot be in the same position as they were layed or they might not get to even hatch
you should get your self some books on breeding if you want to make sure you do everything right, then after that you can figure out what works best on your own experience.