A spreadsheet program may be used for a variety of applications unforeseeable from the developers. Therefore the specification must be generic since it is impossible to predict the numerical range of the calculations. Introducing a limitation would severely impact the usefulness of the program. Therefore to widen the range, floatingpoint is used.
One of the frequent applications is business: accounting, invoices, asset tracking, … This application is fundamentally based on integer arithmetic. This is quite antagonist with the use of floatingpoint.
Yes, accounting is integer, even if you see what seems to be decimals. The smallest “quantum” is a cent (or other minimal currency subunit) and all amounts are multiple of this quantum. Only integer arithmetic can guarantee no rounding is introduced and lead to the “least astonishment” of the customer (for instance the sum of raw price and VAT should be equal to the final price even in the last decimal).
Consequently, all your calculations should be done in integer mode.
This is possible over IEEE754 within a certain range because the 80bit variant offers 64 bits integer part. Some precautions must be provisioned to avoid “integer overflow”:

addition and subtraction are quite safe as numbers in the higher magnitude end of the interval are quite rare

multiplication leaves you 32 bits before entering the “floatingpoint domain”; remember you must scale the result because is a now a multiple of a cent of a cent (i.e. you must drop 2 least significant decimal digits and perhaps introduce a rounding there to revert to your conventional unit, aka. divide by 100)
This may be insufficient if you’re tracking a country budget to the cent or big civil engineering projects. In this case, you need to change your basic unit and accept some inaccuracy in the descriptive power of your sheet.

division is the most difficult operation because the hardware will nearly systematically give you a floating point result; you must first convert to an integer with
CEILING()
,FLOOR()
,INT()
,ROUND()
,ROUNDDOWN()
orROUNDUP()
and do same scaling
Details of computations are described extensively in The Art of Computer Programming, Vol. 2, Seminumerical Algorithms (by Donald Knuth).
As said, all calculations are done in integer mode. Display (fixed point with 2 decimals) and internal representation (integer) are separate, though related, things. Formatting will place the decimal separator where you want it so that sums look like what you’re accustomed to.
In case you find this is a lot of fuss, the alternative is to use adhoc accounting dedicated applications. But check that the developers did things correctly, i.e. didn’t use IEEE754 but implemented multiprecision arithmetic. Decades ago, the COBOL language was invented to address all these accounting issues (because in the end there are legal and tax mandatory rules to abide by). In COBOL, you could require that all computations be done in decimal mode to stick as nearly as possible to real life constraints. But even with this feature, you could botch your results.
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