I have a Chamonix 45F1 and I am currently trying to conquer 120 Velvia 50 using a set of Lee grads but I am having awful trouble aligning them.
I can't see the transition even on a hard step grad. This has to be because I am shooting a 6x12 back, a horizontal slice of the 5x4 frame. Sliding the grad down the holder, looking on the ground glass, just darkens the whole shot.
I keep a small pad of post-it-notes in my 4x5 pack. What I do is place a single post-it on the upper or dark part of the graduated filter. Basically I put the line of where I want the mark at the edge of the post-it note. This way when I stop down a bit, I can see where the post-it note is blocking the image and I slide the filter up or down until the edge of the note falls where I want the filter's density to be.
It works better with wider lenses than longer ones for reasons of depth of field but it does seem to work well either way. Give it a try, it seems to take a lot of the guess work out of where that line is falling. So you are pretty much using the post-it note to as a means to better see the line of where the darker part of the filter starts.
Basically I put the line of where I want the mark at the edge of the post-it note. This way when I stop down a bit, I can see where the post-it note is blocking the image and I slide the filter up or down until the edge of the note falls where I want the filter's density to be.