As used in this chapter, "full-time equivalent (FTE) certificated instructional staff" means the number of staff units determined as follows:

(1) Each employee of the school district or charter school who, as of October 1 of the school year, is contracted to provide services as a certificated instructional employee for not less than 180 full work days shall be counted as one FTE.

(2) Each employee of the school district or charter school who, as of October 1 of the school year, is contracted to provide services for 180 partial days as a certificated instructional employee shall be counted as a partial FTE, such part to be the quotient rounded to three decimal places obtained by dividing that part of the day worked by the full day as determined by the district or charter school.

(3) Each employee of the school district or charter school who, as of October 1 of the school year, is contracted to provide services for less than 180 full work days as a certificated instructional employee shall be counted as a partial FTE, such part to be the quotient rounded to three decimal places obtained by dividing the number of work days contracted for by 180: Provided, That if the normal annual full-time contract for the position exceeds 180 work days, the greater number of work days normally contracted shall be used as the divisor.

(4) Each employee of the school district or charter school who, as of October 1 of the school year, is contracted to provide services for less than 180 partial days as a certificated instructional employee shall be counted as a partial FTE, such part to be the quotient rounded to three decimal places obtained by dividing the part of the day worked by the full day as determined by the district or charter school and then multiplying the result by the ratio of work days contracted for by 180: Provided, That if the normal annual full-time contract for the position exceeds 180 work days, the greater number of work days normally contracted shall be used in place of 180 in the ratio.

(5) No employee shall be counted as more than one full-time equivalent certificated staff unit.

(6) The length of a full work day as used in this section shall be determined by the district or charter school.

(7) As used in this section, contracts to provide services as a certificated instructional employee shall exclude supplemental contract services as defined under RCW 28A.400.200(4).