Achieving (near) exact voltage divider ratio
Resistor series values are designed such that you can select for ratios with an accuracy on par with the tolerance.  For example, the E96 series (https://en.wikipedia.org/wiki/Preferred_number) is used for 1% resistors.  Pick two values from the table and calculate the divider ratio.  If you switch one of them out for an adjacent value in the table, the difference in the ratio will be less than 1%.  This works for any resistor series.  If you need to do better than that, you can use a trimmer, expensive laser-trimmed resistor arrays, or possibly some form of calibration to compensate for an offset divider value.  If you really need high precision, don't forget about the temperature coefficients.