When I make a raglan sweater I use a formula to calculate my cast on stitches. Nobody dislikes math more than me, but this is fourth grade math, so I can handle it.
Measure the neck circumference. This is merely a suggestion since I am not making the opening the size of my neck or I would never get it over my much larger head.
Let's just say that 15" is the neck measurement and my gauge is 5 stitches per inch. 5 stitches x15 neck inches =75 stitches. Simple?
If it is a cardigan, this is pretty close to the amount of stitches that will fit neatly around the middle of your neck, and you might be adding more to that number if you are knitting a button band, so there is wiggle room.
A pullover is a bit different. More wiggle room has to be built into the neck opening or the head won't make it through. So I would say measure where the neckline will lie. This is impossible to do alone, and I never have anyone hanging around ready to do this for me, so I just have to guess, or cheat and look at a pattern (!!!). So let's guestimate that we need 3 more inches, or 18" x 5 stitches per inch =90 stitches. You might do ribbing with a smaller gauge or smaller needle (like 6 stitches per inch) since you will want the neckline somewhat closer to your actual neck, and it will still stretch quite a bit, so don't worry. Try it on and see. If it is too tight, use a larger needle and go for it again. A bit of ribbing to rip out is nothing in the grand scheme of things. Make notes!
Ready for the raglan part?
All TOP DOWN raglans follow this rule, somewhat.
Neckline stitches: 90 divided by 3=30.
Which means 30 stitches for the back, 30 for the front and then 30 divided in half =15 for each shoulder. It almost doesn't matter what your gauge is at this point because you will be knitting INCREASES until the number of stitches on the sleeve parts measure your arm circumference, which depending on how skinny or plump your arms, it is usually somewhere between 12-14 inches.
So counting the stitches on the arm parts, 60 or 70 stitches is when to stop and put those stitches on waste yarn and join the rest of the stitches for the front and back onto a circular needle and continue knitting.
But wait! I have been adding a few underarm stitches to the joining parts recently and never regretted it. I am busty and need that extra width under the arm, so I add 8 between the back and front, on each side. Consequently I must also add those stitches to the sleeve when I come back to the knit them. Knowing that, I go for the skinnier arm measurement (like 13" or 58 stitches ) or I will have way too wide a sleeve and too many sleeve decreases to force into the length in order to make my arms look proper.
So that's basically how I make all my raglans, without having to follow a pattern.
I also make the neck ribbing a few rows deeper in the back than in front, by means of short rows. But that's a fitting thing for my sloping forward back neck, and isn't necessary for more youthful bodies.
In addition, I have finally admitted that hips are a part of my anatomy and I do a series of double increases on each side 'seam' as I knit downward (every six rows or so). This allows for a less grabassy fit. I don't want to know how many stitches I end up with as I reach the bottom hem. Yipes.