/ic/ - Artwork/Critique

Okay, stay with me here

Lets say I have a photograph of a landscape, or anything really, and I want to do a reasonably accurate drawing of it

How do I translate the measurements, the format of the subject lets say, from the image to my page?
Do I just say to myself something like "well it's a rectangle that's almost twice as long as it is wide" and then just freehand a box near that size on my page and try to fit the drawing into that surely inaccurate space?
Or is there a more rigorous way to do it that doesn't go as far actually physically measuring the photo and working out a precise ratio.