How can I calculate DC DC converter accurancy when this info is not available on the data sheet?

For calculating converter acurancy you need to go to the converter data sheet FB figure

Take the minimum voltage and maximum voltage from the data sheet.

With this numbers is easy to calculate the tolerance for example:

MP2332 FB min/max:

(805-789)/ 805 > 1.9%

(821-805)/805 > 1.9%

So the MP2332 Accurancy is +/-1.9%

Note: for the over all accurancy calculation the FB resistors divider also should be taken in account,

If using 1% resistors tolerance - the over all accurancy will be +/- 3.9% (1% * 2 resistors+ converter 1.9%)

A quibble, voltage divider comprised of 1% resistors has a 1% ratio accuracy. It is frequently, wrongly, stated that 1% resistors will give a 2% accuracy.

1k nominal division ratio is 0.5

1.01k and 0.99k gives either 0.505 or 0.495 in either case a 1% error

So a 1.9% reference and a voltage divider comprised of 1% resistors gets you ± 2.9% accuracy.