I have read that the interrupt vector table on ARM processors needs to have the 2’s complement sum of the other vectors at address 0x14.

So looking the current vector table on my board, I see that it looks like this

Addr Values

0000 18F09FE5 18F09FE5 18F09FE5 18F09FE5

0010 18F09FE5 845F20B9

So, if I am correct, then 845F20B9 is the 2’s complement sum. The problem is I cannot calculate this. My understanding is this is how it is calculated.

The values in your calculation are little endian representations of the numbers. Are you looking at the contents of a HEX file by chance? You need to flip the byte order to turn them into their true 32-bit integer values.