B52 - I don't consider myself the penultimate antenna expert by any means. But I have been in the business a long long time, and have a few clues as to what works... and what doesn't. At 860 MHz, 5/8 wave is about 8.5 inches. But that would require a matching coil at the base, not the center. I don't know the value required off the top of my head.
The typical 800/cellular antenna that you're thinking about is usually a quarter wave section, a phasing coil, and a 5/8 wave section. I'd suggest that you locate a cheap used cellular antenna rather than roll your own. But if you really want to do that, you could duplicate the dimensions, and for a receive antenna, it would work just fine.
If you're listening to a strong signal, perhaps just a ground plane would do. 3.4" radiator, and ground plane elements slightly longer. It's easier to make, and you'll probably never notice that 3db loss in comparison to the gain antenna. At 800, all that does is help counter the feedline loss.