Sertraline Taper methods calculation big differences

Good morning ladies gentleman.Thank you for your assignments.I am on 68.75 mg sertraline.i wonder why when I do an automated taper calculation plan online in your site by 1% reduction of occupancy you give me a very tiny reduction of the dose from 68.75 to only 68.15 whereas in my calculation my starting dose of 68.75 mg corresponds by interpolation to approximately 82.61% SERT occupancy. Therefore: Start: 82.61% After 28 days: 81.61% After another 28 days: 80.61% etc. Using the Maudsley curve, 81.61% corresponds to about 60.87 mg. So an exact 1-percentage-point SERT reduction necessarily means approximately: 68.75 → 60.87 mg for the first step. your online taper said that I need more than two decades to reach zero while my own calculation it took only 6 years despite i measure the same interval of four weeks in the two methods.Then why this huge difference exists ? Thank you.