First I would do a sample swatch, and count how many rows there are to the inch. Then decide how many inches from your thumb you want pattern to start. Then subtract 2 inches (or however many inches of ribbing you're going to do at the top). Now for the number of stitches to start with/where to start in the pattern.
Continuing to work in Pattern Stitch between markers, dec one st before 1st marker and after 2nd marker every 13th rnd once, then every 12th rnd 6 times to 42 st. (Shaping should be done on cable rnd.) Inc one st on next following 12th rnd before 1st and after 2nd marker once to 44 sts. Work 2 more rows even.
So... working backwards and calling the final two even rounds before starting the thumb rows 1 and 2...
Row 1: 44 stitches
Row 2: 44 stitches
Row 3: 43
Rows 4-16: 42
Rows 17-29: 44
Rows 30-42: 46
Rows 43-55: 48
Rows 56-68: 50
Rows 69-81: 52
Rows 82-94: 54
Rows 95-107: 56
To place the markers: Divide the number of stitches in half, place the first marker at 14 stitches before the halfway point, and the second marker right at the halfway point.
So... fer instance, to take a ridiculous example, if you got 5 rows to the inch, and you wanted to start at 5 inches about your elbow, and ribbing for one inch. You'd say:
5inches - 1inch of ribbing = pattern starts at 4" about elbow.
4" and 5 rows per inch equals 20 rows.
Row 20 has 44 stitches (according to above table) so you'd cast on 44 stitche, rib for one inch.
44/2 = 22 stitches. 22-14 = 8.
So you'd put markers after the 8th stitch, and then after the 22nd stitch.
To make it look pretty you'd start at row 29, work the pattern for 12 rows, decreasing on the 12th round, repeating until you have 42 stitches, and then following the pattern from there.
Whew! Does that make sense? I really don't think it's as hard as it looks, just took some mathing.