I do not understand why the level of taxation should be linked to morality. I also do not understand why the level of 40% has been picked (well I do really, you're in the UK aren't you?). Here in Denmark the top level of tax is 61%. Taxation is determined by three tiers of government, state, regional and local and if the three add up to more than 61% then 61% is the maximum you pay. So here the question would be 'Is it morally acceptable for taxation to be above 61%'.
Again though, the figure is completely arbitrary. It is a headline figure which, in the UK, takes no account of National Insurance and Council Tax, things which the Danish tax figure more or less include. It also doesn't take account of indirect taxes like VAT (sales tax) which here in Denmark is very high again at 25% while in the UK itis 17.5%.
What people seem to forget is to look at what you get in return. I feel safer and better looked after here in Denmark than I did when living in the UK. Whether I feel 33% better looked after (difference between Danish tax and UK tax, superficially) I really couldn't say.
I must say I get fed up hearing about people complaining about high tax levels (even accepting that I have done this myself in an earlier post). What people should consider is what you get back in return. I especially have little sympathy with vicars in the UK who object to paying £200 council tax. I was in Lithuania earlier in the year and years of lack of public investment is all too evident in the state of the roads, street lighting and so on. I also get fed up with the argument about people's right to choose because inevitably here they are only thinking of things which affect them personally such as health and education. How many people, left completely to their own devices would set aside money to pay for defence, roads, art and combatting alcoholism? I simply can't imagine any family sitting down to consciously allocate money to these items.
Yes, I think that 61% hurts (and you don't have to be a particularly high earner here to be at that level) but I have this feeling that I get a lot back in return and I doubt I would be here writing to you today if I hadn't been able to avail myself of the excellent child care facilities here. Nor would my children have had such a wealth and diversity of experiences had they stayed at home with me.
I think that a large part of whether you approve of the taxation level or not is related to the level of trust in Government institutions. If there is a high level of incompetence and corruption then any percent taxation could be interpreted as morally wrong.