There is not a way to do this currently, at least not the way I think you want to do it. The difficulty in this type of scenario is that the percentages for the remaining prizes would have to change, in addition to their actual values.

For a percentage prize, it's fairly easy to calculate what the value of the prize is based on a known pot. However, this breaks down with a minimum value given to one of the prizes. For example, your prizes would have to be something like:

1st: 50% (minimum of $1000)

2nd: 30%

3rd: 15%

4th: 5%

If the prize pool is $2000 or more, then this is easy to calculate, because 50% of the prize pool meets the 1st place minimum of $1000.

However, say the prize pool was at $1500. Then 1st place would still get $1000. 2nd place would get what? It should be 30% of the original prize pool of $1500, for a total of $450. That leaves $50 for 3rd and 4th place. 3rd place should get 15% of $1500, or $225. But now we've allocated more than the actual prize pool.

You could say that 2nd place gets 30% of the remaining prize pool. That's 30% of $500, or $150. But this doesn't work either, because the remaining prize pool of $500 would get divided into 2nd, 3rd, and 4th place prizes, but they only total to 50% of the pot. Somehow you would have to divide the remaining prize pool among the remaining prizes in an equitable manner according to their percentages. In this case, since they together add up to 50% of the pot, you could double their percentages. But this is an easy case. And as you can see, it gets really complicated really quickly. What if we wanted to have 1st and 2nd places have a guaranteed amount?

The best thing I can suggest is to create your prizes with the 1st place prize being a fixed amount, and the remaining prizes being percentage amounts. Once the prize pool gets to a level where having 1st place prize set to a percentage would cover the minimum, change 1st place to a percentage prize and then change the percentages of the other prizes. This will give the closest approximation during the run-up to the minimum being met.

Let's assume your prizes will be the same as in my example above: 50%, 30%, 15%, and 5%, with 1st place having a guaranteed amount of $1000. Create the prizes like this:

1st place: fixed amount of $1000

2nd place: 60%

3rd place: 30%

4th place: 10%

Once the prize pool reaches $2000, change 1st place to a 50% prize, and change the remaining prizes to 30%, 15%, and 5%, respectively.

The general formula would be to multiple the remaining prize percentages by (100 / (100 - 1st place percentage)). For example, if 1st place was 35%, multiple the other prize amounts by (100 / (100 - 35)), or 1.5385.