How do you get the sanding uniform along the length of the slip,… Also do you have more cylinders you pop in and out of the vice, with various grades of sandpaper on 'em?[/quote]
I’m CONSTANTLY checking my work/slip. The forming of the slip IMO is the most important part. It’s the foundation… if things aren’t symmetrical…bad things happen…and then the good things never happen! 
Uniform sanding: I’m turning the slip around, scraping one side, then the other. Wait, it’s much more than 1/2, I’d say more like 3/4s, and then I turn it around, and do the same thing. I stop frequently to check what’s how I’m doing.
I measure the dip across the entire slip, and I do check my center thickness after I everything is looking, measuring (dip), and feeling right (as in BK’s idea of flex). IF things are going well, the different elements start to come into focus simultaneously, though some faster than others. That’s why a clear calm head is important during reed endeavors.
Too, I check tails and hip area w/ basic measurements, and I check those with how flexible the tails are when I bend them. If the tails are not flexing right (meaning too stiff), and I’m at my lowest number, I’ll ditch the reed. If the tails are too thin then the aperture will be wayyy to open, and trying to fix that (IMO) is pointless. * You’ll spend way more time trying fix that issue then it’s worth, + more than likely, your reed won’t work well, much less sound well.
I also use Tim Britton’s idea of finger pressure on the slip (while sanding; I should say, “fingers”), as I tend to have one edge that has a wee bit more pith. Too, when the finished head is ready for scrape, I finger pressure to control the shape of how the head is scraping down.
I have more cylinders (get them from Tim Britton-he supplies that stuff), and I MAY use a very tight diameter one to A) remove the pith issue I mentioned above…angling the slip will spare the finished gouge B) taking down the thickness at the hips and tails.
Hope that helps! 
