Formula b. will always give a slightly greater lump sum than a.
Example
Maximum approvable pension = £8,000
Maximum approvable lump sum = £18,000 (pension equivalent at say 9:1 = £2,000)
a. would give 5 x £8,000 = £40,000
b. would give £18,000 plus 5 x residual pension of £6,000 = £30,000, total = £48,000.