I also just bought the CB3500 from Bob ( 3 weeks ago), and he's been very helpful.......but what you're dealing with, you shouldn't even have to call him....there's a manual right?
In it it shows you how to lengthen/shorten the stitch. Just turn the knob shown in this picture.
The relationship between the thickness of the leather you are stitching and the length of the stitch, is something out of "stitching 101", or to be more exact, Al Stohlman's " The Art of Hand Sewing Leather".
From that I learned that projects up to 1/4" thick look best stitched at 6 stitches per inch.......3/8" thickness, 5 SPI.......more than that, including your 1/2 or 3/4" thickness, 4 SPI......it's been a good rule of thumb for me when hand-stitching, and it's no different on the 3500.
I've made the adjustment several times already and it's really simple.....turn the knob, do a test on scraps that are the same thickness as you'll be stitching, adjust til you like the length of the stitch.
I'm by far no expert, but we're getting along (3500 and I ) very well, if you get stuck and Bob can't be reached, give me a hollar.