yes its 50% of the total amount payable - which in your example is 50% of £30k.
Obviously with interest included, but the total amount payable will be on the finance agreement and when you have paid 50% of that, including your deposit, that is it, you can VT.
So, 5k deposit on a 30k car, 25k payable (including the optional 10k final balloon payment), when you've paid 12.5k you are free to jog on, which won't be at 50% of the duration of the agreement, but probably is at two thirds or thereabouts.
Usually most who play this, bail at 2 years of a 3 year agreement.
Depending on how confident you feel, many exploit this by having super low PCP contracted mileage, do loads of miles then VT. Whilst everyone assures you that its legal and no come backs, that does feel a bit dishonest to me...
Obviously you need to work your own figures as whilst VWFS interest rates are standardized, the GFV figure seems to fluctuate all over the place. Depending on those actual figures, you might find its possible.
Remember though, you literally walk away with nothing. Zero.
Only worth doing if you are desperate to not pay for a car (ie lost your job or changed job to where you don't need a car), desperately need a different car (ooops baby) or are some sort of chancer or need a shiny new Golf