(A - B) / 5 = Equal spacing between guitar strings
"A" is the distance between your two outside strings.
"B" is the sum diameter of the inner strings.
In your case, with your strings, take the inner four:
18-27-39-49
And you get .133"
So you simply take your space between the outside strings, subtract the sum diameter of the inner strings you're using, then divide by 5. And there you go.
Once you get your spacing number, set your calipers to that number, lock them, and use them to double check your spacing. I like using 3" round leg dividers as they're a little easier to handle, but calipers will work fine.
My preference is to do the string spacing by eye, making very shallow set marks, and then double-checking it with the calipers. But I always go by eye for the final say.
As for "A," set the outside strings by eye, and then check with a machinist ruler or calipers. Start with set marks much narrower than what your final slots will be, and adjust accordingly. Once you have them set, simply measure the distance between the strings with a set of calipers.
The key here is the fret bevels. Too close and you risk slipping off the edge. Too far and the spacing will feel cramped. This a good example of cramped spacing vs. spacing that makes full use of the fingerboard width, and is a nut I cut using the simple formula I mentioned here:
(I actually found that formula on here, I think, from someone else who mentioned it, and it has helped me a lot ever since)
I use an Xacto razor saw for the B and E strings, and the Stewmac nut files for the rest. The razor saw is good for making set marks for all the strings while you're still figuring the spacing out.
The tricky part is even though the math is easy, cutting your marks exactly on point isn't so easy. I always stop frequently and check my spacing.
I usually use a capo behind the nut to hold the strings down. This helps a lot especially on angled pegheads to hold the strings in place while I move them around to dial in the spacing.
Remember on the thicker strings to start small, and work your way up with the files. This is especially true on bass. If you just go right in with the full size file, it will be much harder to have it stay centered than if you started with a razor saw, then used a .016" nut file, then a .028", then a .042", for example.
On bass, for example, I usually use a .016" or .020" nut file for the set mark on the .105" E string, and if it's a little too far left or right, I gradually correct it as I work up to a .050" file, then a .065", an .085" and then finally the .105, at which point it's exactly where I want it.
Working your way up from smaller files will ensure that your final slot is as centered as your original set mark.
On the high E string of course, you don't have this luxury, so go slow, make VERY shallow set marks (just enough to hold the string in place with a little tension on it), and keep checking your distance from the fret bevel before you make a deeper cut.
I *always* end up using the calipers/dividers set to the spacing number from the formula as a guide, not the absolute final say.
There are people who just mark and cut the slots without even having the nut on the guitar and get decent results. My personal preference is to do all the spacing with the strings on, and I always use my eyes as the final say.
So to sum up:
Set your outside strings. Go slow, check your work, and once you've got your outside strings the distance you want from the bevel, cut them close to depth.
Measure between them for "A", and now you're set to do your formula. Set your calipers to that number, put a capo on behind the nut, start spacing your strings by eye, and make some shallow set marks.
One more note: When I have the strings lined up, I use a mechanical pencil to mark either side of the string. Then use a razor saw as close to the center of those marks as I can, put the strings back on, and then take it from there.