I would try to find out when the amount rose from 67K to 72K... in fact, if you can get a letter from the 401K people, you can try to get them to specify exactly when the amount increased.
If it only increased in January, then you will have to use the 67K. If it was more than 6 months and you have this in writing you should be able to use the 72K.
Right now, on Oanda (which is what will be used on the date you apply), the exchange rates are as follows:
$72K = £58,128
$67K = £54,092
£58,128 converts to £16,851 towards the income requirement... so you would only need £1,749 in rental income to meet the requirement.
£54,092 converts to £15,237 towards the income requirement... so you would need £3,363 in rental income to meet the requirement.
You mentioned above that you have £5,830 in rental income over the last 11 months. Using that figure as annual rental income (for the sake of argument)... if you use all of that, the minimum savings you can have and still meet the requirement are:
£16,000 + (2.5 x (£18,600-£5,830)) = £47,925, which is currently $59,350.
So as long as the exchange rate doesn't rise to more than 1.39 dollars to the pound in the next two months, you'll be fine. Currently it's 1.24 dollars to the pound. Just looking at the stats, it hasn't been above 1.39 since before the Brexit vote and hasn't been above 1.30 since October 2016.
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