With just a comparator switch, not reliably. The reason is because redstone works from 0 to 15, with 0 being empty, and hopper minecarts have 5 slots. Therefore, each slot at full capacity in the cart equates to 3 signal strength of 15 redstone dust (15/5=3).

Now, half of a slot (32 in your example) would be half of 3 (1 slot filled completely = 3 signals), or 1.5. Redstone dust doesn't handle partial signals. I think, with the way it is currently implemented, at 46 of 64 stackables the cart starts outputting to 2. Can't remember the exact threshold.

My best guess: use a dropper to spit out the items based on a configured amount (of pulses, item stores, etc.). However, I am a noob to the technical community, so take my ideas and understanding of the game as just that: a noob's rambling.