How do I split a tax refund with my soon to be ex husband? 50/50?

Did you file jointly?

If it was joint, and given that California is a community property state, half is a good starting point. However if filed jointly, its your joint property so you could give all to him or he could acquiesce in your keeping it all. Its up to you. If you remember to raise it, the divorce court may make an adjustment to all your assets (including the refund and in light of how it was divided) when the case comes before them.