Hi,
There's nothing wrong with it.
No. This is a more complicated way to check if it operates....but it's no way to check accuracy, because the error (inaccuracy) of your analog value as well as the error of your ADC will be higher than the error of the DAC.
A more simple way is to send a known digital value to the DAC. This digital value also does not have any errors (analog value, ADC) and thus it's more suitable to check DAC accuracy.
I'm neither experienced with Arduino nor with your DAC.
So to find the problem I need to read Arduino documentation, library documentation, DAC documentation ... and use a paper and a pencil.
You can and should do the same.
Example:
In the DAC datasheet there is a section where you can see an example of how to communicate. Bit by bit, byte by byte...
Compare this with your software.
Another hint:
Use reliable sources of information. Informations from IC manufacturers are way more reliable than some internet document from amateurs. Many amateur document are full of mistakes.
I don't know if your internet document is correct or not.
Klaus