OK, these can be frustrating but the pattern will work out. You got the body and a sleeve so why not the rest. I’ve had the same feeling if it’s any consolation. Usually for me it’s, “A fine mess you’ve gotten yourself into now!”
You have 360sts. Decrease before and after each marker** (8sts decreased per decrease round).
Decrease every other round 25 times (round 1,3,5,7,9,11…47,49.) That’s 200 sts decreased.
Next decrease every round 5 times (50,51,52,53,54). That’s another 40sts decreased.
At the same time (this is important!) when the knitting measures 64cm put the center front 32sts on a holder for the neck. These sts are now out of the calculation of live sts. You’ll be knitting back and forth now. Bind off at the neck edges every other row 2sts once, one stitch once, one stitch once. Since you can really only bind off at the beginning of the row, you’ll be binding off 2sts at the beginning of a RS row, 2sts at the beg of a WS row, one stitch on the RS row, one on the WS, one on the RS and one on the WS.
360 - 200 - 40 - 32 - 8 = 80sts
These yoke finishes are really fun because you’re decreasing so many sts every other round or every round. The work goes faster and faster as you knit (unlike top down at this point).
**ETA Although the directions say to decrease before and after each marker, the photo clearly looks like the decreases leave about a 6 stitch rib at the raglan join. So the decreases would be 5sts before the marker, dec the next 2sts, knit 3, slip marker, knit 3, dec the next 2sts. Anyway you decide to work this will be fine.