Leathercraft Techniques

For all the watch strap masters in the bunch:

I'm looking forward to trying my hand at a replacemnt watch strap for a family member and have decided on 1.2-1.4mm box calf outter with 1.2-1.4mm Zermatt calf liner. I would like to use lin cable thread but would like some guidance on which gauge would be most appropriate. Likewise, I am thinking of using a #9 pricking iron and would be interested in opinions on this as well for the project.

Thanks