Need to add length an width of my desk to nearest half inch.

To the nearest half inch means that the readings you take are either whole inches, or inches an a half.

For example, 1 in, 12 in, 20 in and 20.5 in are acceptable.

1.1 in, 12.3 in, 20.8 in or 21.1 in are not acceptable.

If you get such values, you will have to round up, or down, depending on whatever reading is nearer to the acceptable reading.

Can you post what are the 'precise' readings of your desk and the readings that you intend to give?