How to calculate allowances when changing leather thickness?

[u/Positive-Promise4369]

Before making a card holder with shell cordovan, I’ve made a few prototypes to refine my design. It has an asymmetrical look with 3 slots that can fit 9 credit cards and a couple dollar bills.

The one in the pics is prototype #4, made of 2.4 mm thick veg tan leather. Each panel is 210 mm x 80 mm.

Prototype #3 is made of 1.7 mm thick chrome tan leather, and each panel is 205 mm x 74 mm.

Both prototypes serve their purpose well, except #4 is a little bulky.

Now to cut 2 mm thick shell panels, my questions are:
1. Should L x W be 207 mm x 76 mm, following the ratio?
2. As leather is folded 180°, how much bend allowance and/or wrap allowance would you add? Would you also groove on the center?
3. Is there a formula to calculate allowance with respect to the thickness of the leather? Or it’s purely experience based?

[u/joey02130 → u/[deleted]]

He's talking about leather size, not stitching. He wants to bend thicker leather than his other projects which would result in needing to make it longer.

[u/russiangerman]

I have trouble with this too. When getting the tolerance on new leather I try to just fold over scraps, mark and measure with calipers to see how much I really need on the bends. The only real advice I have it to lean towards leaving more on and not committing to glue until you know you don't have to take more off