BPC only requests 50% of your current overall profits. You will NEVER pay more than 50% while inside the program (if you quit a higher % is requested depending on the contract).
SHARE REQUESTED = ( Current profit x 0.5 ) - Amount Already Paid to BPC
Example 1 - How much do you have to pay here?
In this case we have 165x0.5 = 82.5 - 24 = 58.5 euro to be paid this month
Example 2: What if you lost last month, and this month you made small profits. Will you pay a share?
Since the student had a downswing and lost money (profit went from 652 to 380) he does not have to pay a share. Since he already paid more than 50%, he will of course never be asked to paid anything unless his profits are higher than 652.
In short, never worry. You will not pay more, there are no tricks. Everything is 100% transparent and recorded.