I agree with Davez,
im a dutching man myself, must say i only do three or four.
My findings with which way to go, either perentage on each nag considering odds or say 10 units on each regardless are this.
I found that it is more profitable with the 10 on each, it is also easier to put the bet on and as stated, when the 5th selection at 12-1 comes in, your way in front. This is a system that ive used for a long time now and find that if your bank can handle it, you can make a good little profit.
Just for your info, my results dutching four per race:
av strike rate is arround 60%, i normally double bet after each loss as the high strike rate offers safety, the thing ive noticed though, if you divided the percentages and only put the bets on against odds, sometimes the fluctuations on tote means that even if you win you loose, therefore the "flat" 10 units per nag has worked pretty well.
Good luck