I found the site http://www.ladyada.net/library/rccalc very useful as a machine calculator but I wanted to implement its equations on a spreadsheet which I did. The equations work fine for a capacitor start voltage of 0V finishing at Vc but what would the general equations and spreadsheet look like if the start voltage across the capacitor (Vc) equaled the supply voltage (V) and the capacitor was required to discharge to a required voltage.