Lex was, of course, joking. It was tough to quantify the amount of strength he was using because, firstly, he was not a computer, and secondly, his maximum strength depended on a lot of things.
Individually he understood how much of a boost in strength he would get if he used Domination, or activated his Dragons Heart, entered Super Say- eh, no if he entered his Lotus form where he was tapping into the Lotus' power. But if he stacked all of those states on top of each other?
He literally had no idea how much strength he could exert in a situation like that, because that was not the kind of thing he tested out ordinarily. Displaying his peak strength like that all the time put a strain on his body that wasn't good for him, so he avoided unnecessary exertion.
