Ok, now find the bottom rear of the diaphragm
It just about joins with the psoas muscle, adjacent to at the very least - now, descend from that spot as you inhale. Keep the descending feeling for the entirety of the inhale. Since you mentioned lying down, here's a good one - imagine your spine is a train track, now imagine a train engine on the track - so from that junction, drive the train down towards the sacrum while inhaling, and let it travel back on exhale, but dont let it go much past solar plexus and definitely not as high as the heart. One tip from some master whose name I do not recall said the breathing should not go as high as the heart or as low as the genitals. remember to relax and let go at the xiphoid process and that will help keep the ren loose and free and there will be less tendency for stagnation there. Once the diaphragm motion gets good and smooth then is helps keep the foramina relaxed during breathing - the openings through which pass the aorta, vena cava, esophagus/vagus nerve. That right there will have a good effect on the heart & blood pressure, less constriction on those main trunks. The heart should feel light, buoyant, mildly effervescent as these things become worked out and smooth
Even in mildly complex systems, any outcome is the wrong thing to target, with the process being where the focus should be.