Intrepretation of a clause

The agreement reads as follows:

" the penalty shall be levied at a rate of 1% on the total invoice for the month for every 1% drop or part thereof in the production during the month below 90%"

Suppose the production is at 88.60% , will the penalty be 2%(90-88) or 1.40%(90-88.60). (given the words 'part thereof')

No more clarification is available(as the company never sought for it).

Please tell which treatment is correct