I agree...it would be great to build a water tight case. But we can't
The reason.....statistics.....or lack of.
At present there is neither the political will or physical capabilty to differentiate Scottish trade from that of UK or the variable forms of taxation that are applied to the Scottish population and business.
What we have are GERS which freely admit to being a best estimate.....but what is that?
GERS also fails to take into account the multiplier effect as espoused by prof Richard Murphy and other leading statisticians. As such GERS likely undervalues Scottish tax take and overvalues that of London and the South East by a considerable amount.
We are therefore left in a position where we can never accurately measure Scotlands wealth until Holyrood has full fiscal and taxation control.
What we can say is that in terms of GDP we are similar to other small West European countries.
Given very few of them have the vast array of resource Scotland has, I think it non controversial to say that we are vastly underachieving in union.