Once you've played around with cards to know how every single one of them feels, you can pretty much cut to 26-27-28 cards all the time. Just a false count at the end can even out the odd if you really want perfection.
You can easily hone this, though. Put the deck in new deck order, or separate reds and blacks. Then try to cut it exactly in half and see how far off you are. Adjust the packets until you're exactly 26/26, and then look at it, feel it, and do it again.
This will train your fingers and eyes to 'know' what exactly half the deck feels like. Then you can start doing other numbers - 10, 15, whatever.