I have 80,000$ in my bank account but unfortunately they are oscillating around the £62500 limit for the spouse visa. Sometimes it's 62800, other times it plummets to 61500. So I'm asking, does the FCO just consider the rate on the date of the application or when exactly? The amount had been there for over 6 months as well.
I would believe that the officers have a wide range of discretion regarding the date of the exchange rate they use (and such might be especially the case for a close call such as yours).
That being said there is one example in our archives where (for a student visa) the exchange rate on the application date was used.
You may further take inspiration from this excellent post on Travel SE:
Exchange rates in a visitor application are meant to be reasonable approximations and they will give a reasonable amount of leeway. So while it is not required, an applicant using "best practices" has the option of adding something like...