I have done all the reading you suggested (thanks - some of it was too advanced, but I experimented a bit and figured it out), but I still have two questions:

1) I am currently working with a modelling shot an actor I like did as my base. The posts on aspect ratio you linked (oh Gods - me and maths! The trauma! *g*) all assume that screencaps are your source material, so what do I need to watch out for when I am using stills?

2) I also used another source - a screencap, for which I figured out the right ratio with my trusty calculator :). I cropped it using the neat fixed aspect ratio feature and it still seems too long. REctangular, rather than square. What did I do wrong?

Thanks again for creating this community and being an awesome mod and teacher :)