if you are using a mitre block it will only cut mitres and coving needs a compound mitre, the easiest way is using a template and cutting by hand this is a link for 127mm template. http://www.british-gypsum.com/pdf/Gyproc_cove_127_Mitre_template.pdf
unfortunatley yes you do need to leave it for four days after the final coat there are products around that can speed this up however they alter the finish and durability of it and you will spend a lot of time doing this all again