For the purpose of determining the system value of the operating property of any such company, the department of revenue shall deduct from the true and fair value of the total assets of such company, the actual cash value of all nonoperating property owned by such company. For such purpose the department of revenue may require of the assessors of the various counties within this state a detailed list of such company's properties assessed by them, together with the assessable or assessed value thereof: PROVIDED, That such assessed or assessable value shall be advisory only and not conclusive on the department of revenue as to the value thereof.