The simple answer is that applications relying on Category A submit the past 6 months payslips preceding application date. (the last one not more than 28 days old) should each meet the Financial Requirement.
If you submit a monthly payslip below gross monthly pay of £1550 then the application will fall for refusal.
If you are both sure that the Category A requirements can be met then go for it.
UKVI will not normally make any calculations for either monthly or annualised averages under salaried employment.
For variable income non-salaried employment it's a tricky
position under category A and needs very careful help.
Does your Fiance get paid for holidays ?
How about for sick-leave?