Hi mcbee,
Here is an event I tested
Bets as follows
Pakistan      India
Bet     odds       bet          odds
5.00     1.61     10.21   1.96
12.38 1.97   12.81   1.97
13.21  1.97   17.86   1.65
25.02  1.65
The result after those bets:
Pak    3.00
Ind  -21.77
Then a stop loss back bet was placed
        bet     odds
Ind  15.46  2.50
Resulting in the followng:
Pak   -12.46
Ind       1.42
The setting was to lose 15.00+   so the loss shown 
on India @ 21.77 was okay.
I would be happy to just put half of the bet amount on India.
15.46/2 = 7.73
this would result in the following:
7.73 * 2.50 = profit  11.59
Pak    - 4.73  (    3.00 - 7.73)
Ind    -10.18  (-21.77+11.59)
This loss is biased towards the Fav Pakistan (at time of loss trigger firing) 
which may not be a bad idea.
So I am proposing to amend your formula as follows:
original    ((-profit_loss)*1.065)/(back_price-1)
amended  ((-profit_loss)*1.065)/(back_price-1)/2
Hope you can follow this mess 

I've only tested the one event and I bow to your superior knowledge
and please let me know if I am on the wrong track.
Many thanks
R