Obviously, the left ventricle is not elliptical in crossection, and thus an error is introduced into this calculation of volume.
The next step is to measure the same volumes in systole, and subtract it from the end-diastolic volume. This gives you the stroke volume of one heartbeat. Hopefully, that was a representative heartbeat. Multiplying this stroke volume by the heart rate gives you the cardiac output.
From Bersten and Soni’s” Oh's Intensive Care Manual”, 6th Edition;
The echo stuff comes from Scott D. Solomon’s Essential Echocardiography