Apologies if I have asked this before, I did search my Username and nothing came up.

Last summer I was give a Webley Falcon and a pile of scope mounts to try and get working as time went one I found out that it used an older type of mount so nothing off the shelf in the modern day is going to fit, since then it's been amongst the pondering of can I file out a set to match as I've likely not the funds or luck to get a genuine set up.
What I want to know is how precise do I need to be to matching measurements?
I'll be using mostly hand tools so I don't have the luxuries of fine adjustment that say a mill has and I also don't want to waste what could be otherwise of use to someone so I want to make sure before I start.
I am a complete novice when it comes to scopes as I generally shoot Irons although the further into my 30s I get the more it seems optical assistance is required.