Based on my experience, a 3 quart enema is too much unless you happen to be an exceptionally large person; and any attempt to "invert" your body is not recommended as a means for geting the enema liquid where it needs to be while you hold it.
Relatively speaking, "high enema" means positioning the tip if the enema tube high in the colon, in the neighborhood of where the Descending colon meets the Transverse.
There is plenty of Coffee-Enema guidance and advice here at Uny's forum and other forums across CZ too. I suggest you spend some time "do the due" reading on this topic. The first source I recall reading about Coffee-Enema and high Coffee-Enema was William D. Kelley's One Answer to Cancef.