Glass carriers used in horizontal projection might require temporarily anchoring the negative in place with some low-tack drafting tape such as made by 3M. I secure the upper corners of the negtive to the lower glass with the minimum tape overlap needed. The adhesive touches the base side only of the negative and comes away cleanly afterwards. The areas contacted by the adhesive of the glass and the top surface of the negative can be lightly cleaned with naphtha if any adhesive gets transferred.
To estimate the easel position, first determine the required magnification m. Then for a lens of focal length f,
Lens-to-projection distance = (m + 1)f
For example, if you have a 6 x 7cm negative whose 55mm format width you wish to project at 54” wide to cover a 50” strip of roll paper, you need a magnification of 25X.
Lets suppose that you’re using a 105mm Rodagon G. Then
Lens-to-projection distance = (25 + 1)*105mm = 2730mm
This is the thin-lens approximation and will be a bit low since we’re using a compound-element thick lens. You can get a more accurate estimate by first calculating the focal length of the equivalent thin lens that conforms to your optical system (can be done, but somewhat tedious). For a 6-element symmetric lens at enlarger distances this is usually about 2.6% greater than the actual focal length. So if you substitute the approximate equivalent thin lens focal length of 107.7mm you’ll get a more accurate estimate.
Lens-to-projection distance = (25 + 1)*107.7mm = 2801mm
This is only an estimate, but is usually reasonably accurate within practical needs.