I have not seen anyone that couldn't get out of 3/8 forward, so I would start there, you need to keep going forward with the thumb until your thumb hangs in the hole, then back off 1/8". You can go forward in smaller increments, but it will take longer and cost you more money and you will probably still end up at least 3/8" forward or more by the time you are done.
I have found that 3/8" forward gives you a good starting point and prevents you from knuckling the thumb in just a few shots, after that your hand will learn to relax, then the thumb might start slipping again, when this happens you need to go at least another 1/8 to 1/4 forward. Just make sure your span is right, it may measure out different than it fits, so make sure your fingernails aren't pressing into the back of the finger holes, if they are, shorten the span a little.
One more reminder that if and when you go forward, relax your hand and make sure you don't knuckle the ball or you will injure yourself, so be careful the first few shots maybe do a one step delivery until you get comfortable with it.