> Checking against the datasheet, I came up with ~12.7k to get to 20.4V and the closest I had on hand was 13K
You can measure the 13kOhm resistors that you have and see if there is one with a better matching value. 12.7k is ~97-98% of 13k which is well inside the tolerance of your resistors (it says 5% on the bag).
You can measure the 13kOhm resistors that you have and see if there is one with a better matching value. 12.7k is ~97-98% of 13k which is well inside the tolerance of your resistors (it says 5% on the bag).