Heh.. 38.65% of it (aka. 126lbs).

But you probably really want to know what cross-section of the log would be above water. That will require a bit of calculation. My preferred way to solve it would be a simple program, iterating on the "how much" part until it hits 38.65%. Or calculus, which I have totally forgotten at this point.

But empirically, the top of the log should be about 3.3" above the water in this case. I think.

https://www.mathopenref.com/segmentareaht.html