For context it's an indoor window over a bedroom door, single pane and old building

I restore historic windows and cut glass every day. You need to provide a little more info. Usually the glass fits in a rabbet in the wood sash, so there is about 1/4” wide flat channel for your glass to rest on. Your advice to measure and subtract 1/16 - 1/8 is right but I feel like you’re not quite getting it.

I'm sorry to sound dense, but I'm not quite sure what you mean? Once I removed the glass and old putty there are no slots for the glass to sit in. It's just a bare frame now, there are no little channels that the glass sits in. Do you mean that there are little slots in the frame itself for the glass to fit into?

A picture would be worth a thousand words. Usually when you remove the putty and glass it’s pretty obvious where the new glass rests. Can you take a pic?

Typically speaking, the glass sits just shy of flush inside the casing, to allow for expansion of the wood. The “formula” allows for measuring face to face inside the frame, and then compensating for any expansion.

The glass will be held in place with wood straps that run along both faces of the pane, and then putty or silicone.

Zero wood straps on this one, I thought so too initially as it looks like it from one side. But as I was removing the old glass I discovered it's quite literally just held in with putty

Likely a very old house, or a very frugal repair man was there before you.

I tried to upload but it was removed. After the putty and glass was removed there is a flat area where the glass rests. The inside of the rim that the glass is placed on measures 380 x 425 mm. That's the very smallest measurement of the window, the most inside rim and presumably what the window should be pressed against

Both, most likely

Measure the old glass if it's still in tact and get glass cut to the same size.

Wish I could but it was broken

I've found a very informative site with lots of pictures which have made it very obvious to my little ADHD brain which measurements I need. Thanks to those who replied here, what a helpful sub! Moving house and will be needing more advise I'm sure so will stick around :)

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