[Nexus 5 Accessories] kidgenius:
1. Using 1/4" magnets with this specific coil, you only get about a 40% overlap of the disc and the magnet. I think it'd be much better to have the magnets lined up first, and remove more material out of the middle. 40% overlap is not going to give you much hold. I would doubt that even with 40% overlap, you probably aren't getting 40% of the magnetic force. This is made worse the farther the phone is from the magnet. I'd assume you are getting something like 30% of the force.

2. Not much, and your case will affect it too. Like I mentioned, I'm probably right at 1/8" between the face of the magnet and the phone back itself, after you add in the phone case, etc (2.5mm for the case, 1/32" of wood). As a result, the pull force is only about 1/7 to 1/8 of the magnets capability. Based on some calculators out there, I'm probably at 1lb of force per magnet. In a car, I wouldn't trust it. At the same time though, the wood is super-slick and the case is also super-slick (polycarbonate back). If you have a TPU case, or something rubberized, you could maybe get away with it. Here's how to figure out what you need, at a bare minimum.....

The weight of the phone + case is important, let's assume 150g, if you have a crazy heavy case (unlikely, but we'll go with it):

Force of Gravity: F_g = ma = (9.8 m/s^2)(.150 kg) = 1.47N

Force of One Magnet (from KJ calculator) = F_1m = .93N (1/4" x 1/4" N48 @ distance of 1/8")

Total Magnetic Force: F_m = 4 * .93N = 3.72N

Now, the phone stays attached due to friction. So we need to know the Normal Force (F_n) first to calculate the total frictional force. This then needs to counteract the gravitational force that is parallel to the face of the surface (F_p). We also take the angle the phone will be at. Let's take 10°, though, in a car mount you might be at 0, or even at a negative angle (tilted towards you). So:

F_n = F_m + F_g * sin(theta) = 3.72N + 1.47N * sin(10) = 3.98N

F_p = F_g * cos(theta) = 1.47N * cos(10) = 1.448N

Force of friction is then:

F_f = F_n * μ = 3.98N * 0.7 = 2.78N

μ here is for rubber on wood which is about 0.7. The actual frictional coefficient could be much, much less though. In the case of my particular phone case, which is highly polished plastic on super smooth wood, I'd bet the μ is around 0.25, or less. So in theory, you have a little margin on the holding force of the magnets, as 2.78N > 1.448N. In practice, I think a Factor of Safety of 2 should be applied. In other words, you should assume that the frictional force should be double of the parallel force to ensure a hold. That's also for a stand sitting on a desk. In a car, I would double it again. As Dani mentioned, you would fix this by making the magnets flush with the surface of the wood instead of hidden. Eliminating that 1/32" of a gap gives you almost 70% more force per magnet. Millimeters matter on this type of thing.

All this is also assuming everything lines up nicely when the phone is on the stand. The further you are away from the magnets, the less forgiving your alignment is. That's why I went with 1/2" instead of 1/4". But, I'm all about experimentation (I'm an engineer if you couldn't tell already ;) ) and I'm curious to see what others do. My design here is likely sub-optimal.

In a car, I'd use something like this:

https://www.inventables.com/technologies/suction-cup-tape

And stick that to a stand, instead of relying on magnets.