I can see the rationale for the 1/3 ratio, in case said horse only places: $2.80 returns a profit, as well 2.60 with a rebate.
Myself, I use a 2-1 place/win ratio as a base;
However, place/win bet may not even be advisable-- odds ranges & particular pool pays determine what way to bet:
assuming no pool anomalies, usually-- if a horse places only, the return is highest if instead all the funds are bet to show solely. This is usually so for 5-1 & lesser odds-- though the potential pool payoffs must be calculated: it there's enough of a difference between place / show, the place/win may work irregardless of odds.
btw heavy favorites are still very playable, all funds solely on place OR show, depending on payoff: i have no problem whatsoever with receiving a few cents less(up to about a dollar) than the win price for place or show.
(Note there is money to be made even at these low prices-- I hook them in parlays of 2, 3 or 4 at a time.
I'm just realistic about win %, & prefer cashing over half the time, even if for less-- the only win locks / fairly high % opportunities, besides trainer angles, i'm able to discern nowadays are the dominant run-style horse favored by corresponding bias-- but even then, the prices are so depressed: between today's field sizes, & the widespread past-posting going on-- & to boot the jockeys, being aware of say, a rail & speed + bias, may gun so fast out of the gate that the speed may still collapse)
So, that basically leaves an odds range of @ 6 to 12 to one for the place/win method; odds higher than that, especially in a large field, merit a win bet even less: exotics are called for then-- even if they have to be wheeled with the whole field.
Overtime i have drifted away from the win pool, to me it no longer carries much value-- both from mutuel price & bankroll velocity standpoints. Discounting longshot flyers, if i find 3/4 solid horses with no knocks or question marks, chances are they'll be one of the 3 morning line faves: i can either flat bet them to win & hope 1 or 2 win, squeezing out a small profit...OR i can show or place parlay all of them, multiplying the original bet by 3-6 times, depending on whether i up and pull a profit mid-series or let it all ride. I'm putting much less money in play and hopefully collecting significantly more.