Out of all the prediction sites and old wives' tales (which A LOT of gender predictions sites are), the most reliable I think is the Lunar Theory. Whether it is chance or what-not, there seem to be a lot of successes with following this theory.
Have you tried
www.chartplanet.com? If you can predict with fairly good accuracy when you will OVULATE not conceive, then you can plan DTD accordingly. The instructions on how to go about putting info into Chart Planet is in the Tips/Links Thread at the top of the board.