The accuracy is +/- 5% of the full scale value for elements shown in Table 1, 2, 4 and 6.
For example, if you are using a 500W element, the accuracy will be +/- 25W (5% x 500W). If you want to measure 35W with a 500W element, the reading can be as high as 60W or as low as 10W and still be within specification. If you use a 50W element to measure 35W, the accuracy will be +/- 2.5W. In other words, the reading can be as high as 37.5W and as low as 32.5W.
Other specialized elements have +/- 8% of full scale value (Tables 3 and 5).
It is best to use an element that is close to the power level you are measuring to get the best accuracy.
Click here for a link to all 7/8"line element tables: