Perhaps your clock looks like this:https://flic.kr/p/rwm6zi
replacing the crystal will make a big difference:https://flic.kr/p/rf1dz4
I started by starting new crystals for 2in Mercer gauges which are dead easy because they are flat.
You just need a friction drive like this:https://flic.kr/p/rd8NRX
But other gauges have domed crystals.
These are cut from flat sheet, like the Mercers, and then pressed in with something like this:https://flic.kr/p/rws4DM
Here is the same thing for smaller gauges:https://flic.kr/p/reTGFj
You use 1mm Lexan, polycarbonate sheet, and for 2GBP you have enough for a dozen gauges.
I hold these tools in my Rubberflex chuck which makes secure, repeatable holding dead easy.
It takes a bit of experience to know how tight to cut the blank and how much stretch you need to press in.
But the answer is basically very tight and quite a lot.