Haven't actually validated the cells and formulae (probably needs doing as, for example, 'Florida - OFT'!C6 <> 'Florida - OFT'!C5 and I suspect it should be equal) but a large omission could be sales tax in the states. A small omission would be examples like paying for an FAA medical in the states in order to solo. Are you sure a bike is all you need when in the States? How about transport in the UK, that has a cost. If you have made the decision to drill down to the nearest $25 then you should probably work out food costs more exactly.
Also add 20% contingency.
Otherwise a vey good way to look at it.