Conceive a boy

We're going to start ttc . We want to try to conceive a boy .

Of course we don't mind if it ends up being a girl well be just as happy but right now we have 2 girls and 1 boy and we would love to give our son a brother and it is our last baby and there's nothing to loose with trying. So we want to try the shettles method which says to increase the chances of having a boy we need to have sex on ovulation day and I'm using ovulation tests so when the test shows positive should we have sex that day or the next day for a boy ?