There are no penalties for over reporting the amount in a bank account. Since FBAR has nothing to do with tax (supposedly), adding an extra $100, $200, or whatever is not a problem. It may seem counter intuitive to make sums larger, but in this case, it doesn't hurt.
If the balance were $150,000, and only $149,900 were reported, it probably wouldn't be the end of the earth. For normal accounts, it's the details of the account (where, account number, etc.) itself that are of interest.
........as from 2017 UK banks are going to be feeding the US with details of balances and interest.
Consider looking at it from the other direction. You're also ratting on the bank when you file an FBAR or 8938. Enjoy the fact the banks fear your reporting as well! Well, except when the banks take the easy way out and close all US Person accounts.
The daily balance on the monthly statement in the checking account doesn't show the maximum value of the account that day so would the bank, HSBC, report the daily ending balance to the US or the highest balance during the day, which it will have been at for several hours?
'Some say' it's second by second, 'some say' daily balance, 'some say' end of month balance. IMHO, that's a line by line that yields the highest balance at any time during the year. Or, simply do as you understand what the instructions for FBAR request.
On an aside note, HSBC are now starting the electronic searches, looking for US indicia on older accounts. Notification and request for a Form W-9 comes in a 8 page letter. And yes, each page is about the request. The recipient, if a US Person, is directed to the IRS site to download the W-9, and is required to hand deliver it to the bank with an additional signed page indicating "the information is to the best of your knowledge". HSBC also urges recipients to go to an HSBC UK site where a great amount of detail is given, along with work sheets for computation of time spent in the US (the 3 year test for presence), guidance for determining if you are a US Person (a very tricky area for the bank to be advising on), and more.