Tax office has a different rate

I note what you say about believing that French tax law does not fix an exchange rate. Interestingly, our local tax office is given an official exchange rate at the 31st December each year.
This is usually available sometime during March (in plenty of time to do your conversions!).
I am informed by the tax office that the rate for 2007 is 1.363605373. This is a more interesting conversion rate than the one you have published. Where did you get your rate from?
ME

The exchange rate that, as you say, you were given by the tax office is for that one date only – December 31, 2007. However, the exchange rate we gave was the average rate for the year, the average exchange rate being the one needing to be used for converting the whole year's income from sterling into euros.

And for the sake of being fussy (!), the French revenue services do publish exchange rates at each quarter-end date, but not averages of the year.

If you missed the April edition, the helpsheet is available from the site by clicking here, priced €5.