Thanks very much ajcgi - this seems to be working. I'm not fully understanding the averaging to get the apex values, and actually the auto georeferencing is pulling in values off the geotiff that are showing a bounding box in TG but no terrain detail. If I manually georeference, then I get a result. I need to work with this some more and get a better understanding, but I'm definitely getting some results now, and much appreciate your help.
Thanks to Ariel too.