Argh. I just got bitten by the ln/log issue again and only now remember the conversation! Fool me twice, shame on me!
Note for Future Phil - when they write ‘log’ they actually mean ‘ln’
Also, the vollsplitting is calculated by calculating tax at half the income, then using the tax rate from this applied to the full income.
I found my original thread: Do you know your marginal tax rate?