If you are using your spinnaker in under 12 knots of true breeze you could mount the halyard block all the way at the top of the mast. The backstay helps to support the masthead above the hounds and in light to moderate breeze that should be fine. This would allow you to use a much larger spinnaker for more power. If you want to play it safe and not take a hit in your rating then the block should mount a few inches above the headstay. Of course you could get really creative and have both, that's what I did.