That's right.
General damages (injury) is in one chunk so past and future all in together.
It can't be deducted from future special damages, so if you're getting a sum for your treatment to continue for example nothing can come off that.
The 25% is a maximum, so it can be less. It's worked out by reference to the actual time spent by lawyer at their hourly rate and the percentage set out in the CFA itself (which will be based on a risk assessment carried out at the start of the claim). Your lawyer should be able to give you a breakdown of it.
Example...
Total compensation of £10,000 (for ease we'll say that's all made up of generals plus past specials)
Success fee percentage - 50%
Costs on hourly rate basis - £4000
Success fee would be (£4000 X 50%) = £2000. VAT then added and so total of £2400 is payable by you.
Example 2...
Total compensation of £10,000 (for ease we'll say that's all made up of generals plus past specials)
Success fee percentage - 100% (must have been a very risky looking case at the outset!)
Costs on hourly rate basis - £6000 (it was riskier as less straightforward so probably more arguments about liability for example)
Success fee would be...
(£6000 X 100%) = £6000
However, the law is that success fee must be capped at 25% and so only £2500 is payable (including VAT)
Hope that helps.
You should have been given reasons for % at the time you sign CFA (no win no fee agreement).