OK, had a bit of time and a spreadsheet and came up with this...(apologies for the font but i need fixed-pitch for this)...
        
         Where:
        
         P = Win pool
         D = The current odds
         x = the proportion that you want to reduce the odds by. 0.1 = 10%. For example, if the odds are 5, 0.1 reduces this to 4.5 = 5 - (10% of 5)
         n = your bet
        
        
         n = Px
        	 ----------
        	 D - Dx - 1
        
         Using a Win Pool of $10,000 and x = 0.1, I got...
        
         P	   D    T	  n	     P+n   T+n	   Dn
         ----- -- ------- ------- -------- ------- -----
         10000 20  500.00   58.82 10058.82  558.82 18.00
         10000 19  526.32   62.11 10062.11  588.43 17.10
         10000 18  555.56   65.79 10065.79  621.35 16.20
         10000 17  588.24   69.93 10069.93  658.17 15.30
         10000 16  625.00   74.63 10074.63  699.63 14.40
         10000 15  666.67   80.00 10080.00  746.67 13.50
         10000 14  714.29   86.21 10086.21  800.49 12.60
         10000 13  769.23   93.46 10093.46  862.69 11.70
         10000 12  833.33  102.04 10102.04  935.37 10.80
         10000 11  909.09  112.36 10112.36 1021.45  9.90
         10000 10 1000.00  125.00 10125.00 1125.00  9.00
         10000  9 1111.11  140.85 10140.85 1251.96  8.10
         10000  8 1250.00  161.29 10161.29 1411.29  7.20
         10000  7 1428.57  188.68 10188.68 1617.25  6.30
         10000  6 1666.67  227.27 10227.27 1893.94  5.40
         10000  5 2000.00  285.71 10285.71 2285.71  4.50
         10000  4 2500.00  384.62 10384.62 2884.62  3.60
         10000  3 3333.33  588.24 10588.24 3921.57  2.70
         10000  2 5000.00 1250.00 11250.00 6250.00  1.80
        
         Where:
        
         D = initial odds
         n = bet
         Dn = new odds (all other things being equal, of course)
        
         also:
        
         P+n = win pool after placing your bet
         T+n = total amount bet on the runners after placing your bet
        
         Does this make any sense at all?
        
         Steve