1. Remove the fresnel. Some say that it is impossible to use a loupe together with a fresnel anyway.
2. Correct for the focus shift by shimming the groundglass by 0.5mm. I have done this by cutting 5 frames made of acetate sheet in the size of the groundglass with margins in the size of the rebate the goundglass is sitting in. These sheets of 0.1mm thickness are made for use with overhead-projectors.
I have checked the accuracy with a collimator on the cheap following a suggestion made in the German LF-forum here.
It works as follows:
You need a spyglass focused on infinity (a star will work) and a torch.
First look with the spyglass through the lens of the camera onto the groundglass and focus the camera. You will clearly see the markings on the groundglass. There is a cross in the middle or instance. These marks are on the mat side of the glass. So you have a correct reference.
Then load a holder with an old negative. on which you draw some marks with a pencil. Put this under the groundglass and look with the spyglass and the torch through the lens. The distance between the spyglass and the lens doesn't matter so there is space to place the torch appropriately.
You will see the pencil lines you have drawn clearly in focus. I fact you can see the grain of the negative in focus if it's HP5 ore something like that. Otherwise you will have to work on your shimming.
The focus shift of about 5/10 mm was mentioned somewhere in the bashing-thread. It seems to be correct.