i have run out of wool to complete a project and had it laying around. Thought Id try to complete with a contrast yarn but now I cant find the pattern for it. It has been suggested i pick up from armhole and knit top down. However I don’t know how to calculate making the sleeve cap. Can anyone advise. It has been suggested I will have to use short rows . I have used short rows before on collars but not in sleeves so I also wouldnt know where to start. I would guess it would have to be from the middle of the shoulder seam in the sleeve pickups. Would i start from there picking up or from underarm. does anyone know where this might be explained or better still actually done it successfully. I’m loathe to throw the project as for my other half. 
Welcome to the forum!
I’m not sure I understand the title. Have you already knit the front and back of a raglan sweater or do you want to change to a raglan?
Can you remember the name or designer of the sweater pattern?
Here’s a nice video on shaping sleeves from the top down starting with a pick up of sts at the underarm.
https://www.youtube.com/watch?v=VdMl1PXkq8c
It may help if you post a photo of the sweater pieces so far.
I’ve used this tutorial before and liked it. If you like text and illustrations rather than video this is a good one
On the initial pick up row, if you knit with your working yarn and an additional embroidery thread in a contrasting colour (holding both together to knit the pick up), drop the embroidery thread when you finish the pick up row. Afterwards you can retension the pick up row which might be loose with enlarged stitches. The embroidery thread helps to show exactly which stitch to re tension as you work around the armhole beginning at the last stitch picked up and moving slack into the next stitch until you reach your beginning tail, at that point all the excess yarn moves into the tail which will be woven in and cut. The embroidery thread can be pulled out in one smooth yank. This retensioning requires the sleeve cap to be complete, don’t try it any earlier than that as you need proper stitches and a few rows worked on every picked up stitch to stabilise them.
I should add, this is for top down, seamless, set in sleeve, not raglan.
Yes, I was about to say–raglan and set-in are 2 different sleeve finishes. Raglan has diagonal lines from the neckline to under the arm, set-in sleeves are seamed around the armhole (or picked up and knit into the armhole). So they need completely different techniques.
Thanks for the link to picking up for a set-in sleeve!
I’ve knitted the jumper except for sleeves so the raglan shaping is in place. I know for the straight sleeves the sleeve other than calculating the decreasing to wrist size is easy but the raglan sleeve needs a cap at the top before you begin the straight knitting for the sleeve. I’ve no idea what pattern I used unfortunately. I have regular magazines with loads of patters as well as collecting old mags. I’ve tried looking through all of them but still not found it. I’ve no idea how to calculate for the cap .
Are we talking about separate pieces, so the sleeve will be sewn along the raglan lines? Or is the body all knit and you need to pick up along the armhole area and continue with the sleeves? Maybe a photo of your knitting would help clarify what you need.
Ann Budd has two books about different sweater types with numbers that may be helpful.
Still, you may have to experiment with the sleeve cap. If you knit cuff up making the desired increases, the sleeve cap from the underarm to the neck will copy the slant of the front and back raglans. You can use that as a guide to the sleeve top.
Where to stop and how many stitches to work along the way will require some measurements. You might use a raglan sweater that you have that fits well and take the measurements from that. Then use the stitch and row gauge to work out the number of stitches needed as you shape the sleeve.
Of course this can also work top down, too.
I’ve found that drawing this out life size using the measurements and cutting out the outline helps. Consider this a pattern that you can hold up to your knitting to keep on track.
I haven’t found a tutorial for short rows sleeve caps on ly for short rows in a collar or as darts in main body. I was hoping for a link from someone. I can understand the logic abd how it works in principle but if I only add I stitch each time it would be too small as its super chunky and do I pickbup from armhole each row or sew in after making stitches. Thanks for response though. 
This video sets it out pretty nicely for sleeve caps.
https://www.youtube.com/watch?v=VdMl1PXkq8c
This video works slightly differently, from the shoulder as beginning of round but also uses German short rows.
https://www.youtube.com/watch?v=37eiSCoVz5k
As the pieces you have are seperate (I think), might it be easier to make a seperate sleeve, either top down or bottom up and seam it in. Can you work out the row numbers and decrease rate of the raglan sleeve top by counting rows and decreases on the the front and back, to produce the same raglan slant and length?
Some raglan go to a point, at the top/neck but some have a number of stitches across the top for the neck band, do you know if you need some stitches there to make the neck big enough?
Thanks for the link.