Where do the halyards come out at or near the top of the mast? On the aft side of the mast you should have your main halyard and possibly the accessory halyard (if it is there then it should be used for the
mainsail boom topping lift). These halyards are probably side by side. On the front side of the mast you have your
furler halyard and probably above that the spinnaker halyard.
Use the halyard coming out of the front side of the mast and above the
furler halyard for your spinnaker. Hopefully that matches your
deck level labels.
You definitely should not be using a halyard that comes out of the front of the mast as a topping lift for the
mainsail boom behind the mast. Hopefully the accessory halyard is side by side with the main halyard and can be used as the topping lift. If that’s not the case and the accessory halyard comes out the front of your mast, then where does it come out? If beside or below the furler halyard then it should not be used for the spinnaker. It could be used as a topping lift for a spinnaker or whisker pole.
If the only halyard out the back of your mast is the main halyard then you have no mainsail boom topping lift. Weird if so. On your size of
boat there is likely a second turning sheave mounted beside the main halyard sheave, so you could add a boom topping lift.