Per my post above, there is no de-facto residual percentage because a residual value primarily depends on a base depreciation value and 3 variables:
1) Initial value of the car
2) Length of lease
3) Total mileage allowance
This topic is about the base residual value being increased. Factor in 3 variables that have considerable variance and you will see people with residual values varying greatly. To illustrate with an example (these are not real numbers)
Base depreciation = £1,000 per year
1) Initial value of car = £10,000
2) Length of lease = 3 years
3) Total mileage allowance = 12,000/year or 36,000 total
After 3 years, the base depreciation would total £3,000, so a base residual of £7,000. If that base depreciation is based on 10,000 annual miles (30,000 miles in 3 years), this example would exceed that by 6,000 miles (3 years at 12,000 miles). Let's say depreciation of miles over base base is £.50 per mile, this would equate to £3,000 (6,000 X £.50). This would bring the residual down to £4,000. (I reckon mileage over base depreciation is built into monthly costs, I'm just line-iteming it here to illustrate)
I'm using simple/extreme numbers to make the point that these 3 variables have a big impact on residual such that there is no simple, matter of fact "residual percentage".
N.B. I'm not a finance guy. I don't know all the workings of this stuff. I only speak to what I've experienced and to my limited understanding.