I'm starting to make a pattern from the newest issue of knitty.com. I always like to know how a pattern is going to fit before making it -- and sometimes I redo patterns completely. So I had to devise a way to redo patterns or, at least, compare them to my other patterns. Here's my formula for converting a pattern to raglan.
First: Determine how many more garment stitches than sleeve stitches there are.
BB = number of garment stitches (from pattern) below the underarm divided by 2
SS = number of sleeve stitches (from pattern) near underarm seam (Include stitches belonging to underarm seam.)
Add 2 (or 4) to SS -- 2 if there are an odd number of stitches in the raglan seam, 4 if an even number. (I arrange the raglan seam so that just over half of it belongs to the sleeve.)
D = BB – SS
Second: Measure your neck. Multiply by 120%. Multiply by the gauge. Call this number N.
OR: Second: Count the number of stitches at the neckline. Call this number N.
Third: Calculate the number of stitches at the neckline (so that B – S = D).
S = (N / 4) – (D / 2) -- number of sleeve stitches at neck
B = (N / 4) + (D / 2) -- number of garment-back stitches at neck
The type of neck chosen determines F, the number of garment-front stitches at the neck.
B and S should be made into whole numbers -- and possibly tweaked further to fit in with the ribbing and/or garment patterns.
Fourth: Determine U, the length of the underarm seam, and R2, the number of rows with raglan decreases/increases (decreases if knitting from the top down, increases if knitting from the bottom up). (One choice for U is BB/6. Another choice is the value of U from the pattern.)
After U is chosen, then R2 = (BB – B – U) / 2. U may have to be changed in order to make R2 a whole number. Also check that 2 * R2 < R, the number of rows above the underarm seam. (Raglan decreases/increases are usually taken every other row – and sometimes every fourth row. For this reason, 2 * R2 needs to be less than R.)