You're on the right track i think. Reduce bets or eliminate the under value selections will improve your place return big time, in that respect its no different to win betting.
Off the top of my head:

Grade your selections as 100%, 90%, 80%, 70%, or 60% chances of placing (in a theoretical 8 horse race). Just base it on your confidence level. Now adjust this perceived probability of placing by the acutal number of starters.

So, a 90% chance in a 12 horse race is 0.9 x 8/12 = 60% chance. A 100% hope in a 17 horse race is 1 x 8/17 = 47% chance. So adjust your bets AND minimum value price to these calcs of percieved value.