Leathercraft Techniques

Hello  I would be really grateful for some help from anyone on here who makes watch strap please. I am trying to design my own templates for watch straps and I am struggling as how get an accurate sizing guide.

I have looked at a few other makers sites to view their sizing guides and to see how they have come up with the sizes of the straps from that of a wrist measurement but I cant quite seem to work out how this is done.

For this example I am going to use a wrist measurement 17.5 cm which in the extract of one particular makers size guide would indicate that the wrist measure is comfortably in the size range for a 115/75 strap and a lug to lug measurement of 40 mm.

Wrist Size Strap Size

(16.5 - 18.5 cm)  Regular length (115 - 75) when suitable with watch face with a lug to lug measurement between 35-42 mm.

Given that 17.5 cm is in the middle of that size range it should then in theory be the same as that from the middle hole to the point where the pin bar meets the bar of the buckle (inc the watch lug to lug measurement) as this being the point where both shall meet when on the wrist. My understanding is the the buckle being slightly off centre of the wrist so not to dig into wrist when the arms are on a flat surfaces.

If the length of short strap is 75 mm then should the middle hole is on the long strap also be at 75 mm? In the example picture I have marked the centre hole at 45mm from the point of the strap (as this guide used by the same maker so not to have too much of the strap curling around the wrist after it has been put through the keeper).

When this done on the design example pic I get a measurement of 195 mm so 20 mm difference from that of the wrist measurement. Is this to allow for of the thickness of the leather as obviously it is thicker than paper meaning when wrapped around the wrist there will be a greater surface circumference or is there something I have overlooked? Either way I would like to try and get my head around things to understand the process.

I also noticed that 7 holes on the strap seems to be the most popular yet looking at the size charts that many makers offer the wrist size range would generally indicate more towards 5 holes at 5mm spacing despite the fact their work often show straps with 7 holes.

In the example pic I have used of 6.5 mm spacing between holes as that the size of the strap punch that I have however at the bottom I have included the spacings based on 5mm. 

I understand that different makers will have their own adaptive process on how they come up with their numbers but I was hoping that someone would be able to explain if there is a correct method or how they worked out their sizes? 

I would be most grateful on and light that can shed on the subject.

Thanking you all in advance