@feelinghthelove:
To answer your question: SMP is effectively paid daily for all days of the month (including weekends, public holidays etc) so you should be able to count the number of days in the month and multiply by 146.68/7.
However - even if you get paid on the last day of the month that pay cheque might represent pay for a different period (e.g. 2 weeks arrears, 2 weeks advance, although being paid on the very last day of the month, would suggest the period is for the month in question) your HR team should be able to tell you in theory...
Were you £60 down on the gross pay or the net pay that you received? There are a lot of reasons why the net pay might be down e.g. pension contributions, student loan, attachment of earnings other work benefits. You would need to provide a lot more information to accurately calculate your net pay within +/- £60. Your employer is certainly best placed to do that.
There is a calculator at www.maternity.money that shows take home pay, but it is still an estimate and so may not be accurate to within £60.
If it's your gross maternity pay that is lower (before any deductions) than the .gov maternity pay illustrator that seems more strange...
Can you tell us how much you actually got in which month?
@MumApr18 - calculating average weekly earnings is actually done from (roughly, it's a bit more complicated) 15 to 23 weeks back from due date, rather than the 8 weeks directly before due date. This is mandatory for SMP, so your employer can't choose to do different.