Properly sizing Pocket Watch crystals can be a bit of a challenge over the internet. It's hard enough to do if you have the watch in your hand but getting a perfect fit by mail is tough.
A perfect fit on a PW crystal is were the crystal just "snaps" into the bezel. Kind of " Goldielocks " Not too tight, not too loose, Just right. That's because there is an undercut in the bezel that you can't accurately measure and must be taken into consideration for that " snap " fit. Also , as shown in your photos, an accurate measurement must be taken first , to establish a starting point . I can ashore you that your bezel does not have two different measurements from top to bottom and side to side. That is if it is undamaged, and your appears not to be.
As for the conversion from Linges to MM this chart will help most people , but yours is off the chart. In your case just order by MM.
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Don't forget to add the little bit for the under cut.
Here is another chart for the oversized crystals offered by a supplier on Ebay
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Next you must have hand clearance. That is called " dome " . Since yours is an open face it will be much easier. It is almost impossible to do remotely for a Hunter ( closed cover ) watch as too much dome and you can not close the cover.
Use this chart for " dome "
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The higher the dome number , the flatter the crystal.
The last decision is thickness and that is based on the style of your watch.
I hope this helps you, even if yours is not on the chart, to understand the numbers on the crystals.
Later production crystals will also state the size in mm with an assumed decimal point . Such as a 507 number on a crystal means it is 50.7mm in diameter
That same seller on Ebay currently has this crystal at 25 2/16 Lignes that he says measures 56.68mm with a dome.
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The long and the short of all this is , contact the suppliers with these large 23 - 25 ligne crystals and give them your dimensions , they will work with you to find the right one for your watch. You may have to buy a " range " to get a fit.