After reading the paragraph “Huiyin and Perineum coordination” on this page I decided to stop contracting my pelvic floor muscle while doing MCO until I feel that my lower dantian starts overflowing. It is not necessary yet and only makes the MCO exercise unnecessarily complex. I will devote my evening meditation now to belly breathing , while staying aware of thoughts, emotions and sensations.
So I started this tonight, spent about 45 minutes doing belly breathing. Concentration was decent.