I don't think you're doing it right. Loosen the 19mm bolt; some say half a turn, some say until it's finger tight. Then tighten the tensioner adjustment nut -- it's the brass colored nut on a vertical axis behind the power steering reservoir until the indicator on the tensioner body (above and behind the 19mm bolt) is at the max point on the scale.

I don't know what you're going by applying force to the pulley, but I'm pretty sure it's not the right way to set belt tension.

Sixto
91 300SE
81 300SD