Setting up a decent red light for your navigator is all well and good, until it dies, won't stay in place, etc. We installed a red interior light on a flexible shaft, which was sweet until it didn't work, and then I was sad. So for a bunch of years I used a headlamp that I covered with red cellophane, then I had one with a red auxiliary light, which worked well. I'm partial to the Petzl brand, but I'm sure lots of others have a red light, too -- Petzl tends to be on the spendier side.
Have your navigator get a lap desk. I just looked for an example of what I used to use, and can't seem to find anything like it now, but it was plastic, had a storage compartment for pens, calculator, rule book, etc., then a clipboard clip on the top with which to hold the route book in place. Even if you can't find something similar (that is, with storage), any lap desk is better than none, IMO.
Put a piece of velcro on the bottom of the calculator and another piece on the lap desk, to keep it in place. I've seen people do this for their pens/pencils, too.
Last, I infer from your post that you and your navigator haven't done this before, and your navigator is your SO. TSD can be super fun -- the teamwork is what I enjoyed most about it. Math dorks, especially, dig this stuff, as you can imagine. But be prepared for the small chance that TSD literally makes your navigator sick: they may make you stop, to keep from puking, they may be so nauseous that they have to get out of the car, lie down, close their eyes, and keep still. It's worse on pavement (more g's) than gravel. Some people can take pills, others use a transdermal anti-nausea patch behind their ear. Others simply try navigating, are made miserably car sick, and either become drivers, or don't rally. :-)
Edit: handy link to a GRM thread about TSD: https://grassrootsmotorsports.com/forum/grm/learn-me-tsd-rallying/4480/page1/