If you accidentally lost some sand when you decanted tyhe NaCl, how would the % salt calculation be affected?

Unchanged.

Explanation:

Hello,

In this case, for the experiment you performed, as you previously removed the sodium chloride, its % salt calculation will not be affected by the lost of the sand. It is important to remember that such experiments must be performed carefully as the committed error is a high-impact factor regarding to the experiment's performance. It is important for you to remember that to separate the sodium chloride, it earlier dissolution in water allowed to be dissolved leveraging the insolubility of sand in water, thus, it was decanted easily.

Best regards.