I have found this varies considerably,but it really doesn't seem to be critical and the rule of thumb I have been taught is that it should be no higher than the center wheel pivot location. I have some that I am sure are factory installations on Self Winding rotary seconds beat models with Howard pendulums that do not comply with this rule and seem to run just fine. I am sure there is a good engineering design reason for the factory location. I think the arc of the pendulum will be affected to some extent and the power requirements to drive the pendulum will be changed depending on location but most American clocks are so overpowered they seem to function even if this is not located exactly as originally intended.
Yours sounds like it is a mercury pendulum from your description. Perhaps you can see some slight wear marks on the rod that would guide you to the original location.