I don't know anything about Gimp...sorry.

But your approach seems ok to me - try to keep the tripod level as you suggest.

You need to choose the best lens for the shot (of your 2 lenses the zoom is most likely) - only you know what that is when you get to your scene and check out the framing.

I'd also leave room for cropping so I'd 'overframe' the scene by maybe 5-10%

Good luck - hope that helps...and love to see some shots!

best!

marko