Is the height of the front sight, from where it comes out of the base to the top of the sight, always the same on the SKS when it's shooting properly at 100 yards? My sight rises 7mm from the base and it shoots just a tiny bit high at 100 yards...does anyone know if 7mm is standard or if it varies from SKS to SKS?
We can't tell you because it varies according to each individual rifle, each individual shooter...and to a much lesser extent also the ammunition "flavor" (the cartridge may be 7.62x39, but the bullets can vary in weight, along with the gunpowder amount and speed of it's burning, all will affect exactly where the bullet will go). In an SKS, you do not have to worry about the ammunition being a factor for reloading, I only mention it because in future you might have a longer range rifle with a sweet barrel!
See, older rifles tend to have slight differences between each other, right off the assembly line, because they didn't have super-precise robotic machines making them...they were made and assembled by skilled tradesmen who honed their craft through decades of hard work. Also, you the shooter will hold the rifle differently than I might, look at the sights slightly differently, your shoulder might move less or more than mine, because of differences in weight and muscle. You will likely pull the trigger differently, at a slightly different angle. The barrel may have had manufacturing stresses on it, or it might have been dropped or hit at some point.
Yet despite all of that, you can get the very best out of your rifle, any rifle, by zeroing the sights.
I strongly suggest you use a big piece of paper, with a small orange/red target of say 1 to 2 inches in size, on a target just 25 meters/yards away. Put your rear sight slider on the "2". Shoot carefully, as accurate as you can, resting the front of the rifle on a sandbag, or your backpack. Use your left hand to support the rear stock into the shoulder. You should be rewarded with a cluster of shots perhaps no bigger than 2", and probably nowhere near where you were aiming. That cluster is what we call a "Group" or "Grouping". Once you consistently see a nice grouping in the same place, you will adjust the front SKS iron post, so that this Group will be right where you were aiming!
Actually, at 25m, you will probably want it about 46milimeters higher than your point of aim, with your elevation sight set to 200meters. This should reward you with getting your "Group" right on target at the distance of 200m, as well as any other distance you slide the range slider to.
These aren't sniper rifles by any stretch of the imagination, but once adjusted, they are better than many give it credit!