Best method to cast on for 400 stitches

[tigger61568]

I saw a fantanstic scarf on Knitty Gritty yesterday that I wanted to try. However I have to cast on 400 stitches. What would be the best method to use? I have to purl the next row after casting on.

Need size = 10-1/2
Yarn = 1-3-4oz/50g/108yd
Gauge = 20sts and 36 rows = 4"

I was thinking the long tail method...but if I have to use 1/2-1" per stitch...wow! That's a pretty long tail.

Can someone help? Thanks!