How to calculate under or over exposure using these values TTL meter (Lux), Shutter speed and ISO/ASA

I would like to get into film photography as a hobby, I own two 35mm film Cameras which both have broken TTL meters, they are common Fujica bodies and I would like to replace the (battery damaged) TTL meter system with a new sensor and microcontroller as a project.

My aim is to use the inbuilt indicator needle +/- to indicate if I am over or under exposing the film. The issue is I cannot for the life of me find a formula or work one out.

I have found two answers that have helped but not enough to work out a formula that works.

What is the difference between luminance and illuminance?

Is there a formula to calculate ISO according to Shutter speed?

The light is measured through the prism and that appears to need an extra value called reflected-light meter calibration constant. I cannot find this for Fujica but I’ve tried using the Pentax value which is 14.

To be able to calculate EV I would need f/stop, however, I can only access the following values.

1. Shutter speed
2. ISO/ASA
3. LUX

It must be possible to obtain if I am over or under exposing the film from these values as the cameras did it before they broke. It would be fantastic to find out what the formula/calculation would be? It might be a simple case of my Google-Fu failing me due to lack of knowledge.