I usually back them to "take out" a certain amount using my price (if shorter than Tote) or Tote Odds (if VV, but don't usually take what I consider to be under the odds).

To arrive at the nr. of units to bet I simply divide the amount I want to "take out" i.e. collect, I just divide the takeout figure by the expected dividend.
For example, if I want to take out $12.50 and the ****. divvy is $3.90, my stake would be $3.20, or $3 when round to the nearest 5o cents.

The take out figure is winnings & stake.

Gerry