Puntz, I think the following is what your looking for:

If you have 3 horses with odds 3.50, 4.00 and 5.00 and you wish to dutch to 25 units profit you have to stake the following:

3.50 = 27.0270 stake = 94.5946 return = 25.00 profit
4.00 = 23.6486 stake = 94.5946 return = 25.00 profit
5.00 = 18.9189 stake = 94.5946 return = 25.00 profit

Total Stake = 69.5945 or 278% of profit target.

For the stake to be 80% you'd need odds (as an example) as per the following:

4.00 = 11.2403 stake = 44.9612 return = 25.00 profit
8.00 = 5.6201 stake = 44.9612 return = 25.00 profit
14.50 = 3.1008 stake = 44.9612 return = 25.00 profit

Total stake for this scenario is 19.9612 which is 80% of your 25 profit goal. Note in this scenario you have approximately 44% of winning the bet based on the odds.

The above can be easily calculated in excel, took me some fiddling around before I remembered how to do the equation but got it all sorted now. Email me if you want to discuss further or just simply want the equation and I can help you set it up in excel. I find its better to know how an equation works rather than simply using a downloaded calculator.

You'll find my email address in UB's 100 units profit thread in the system section.