EO Traveller Data Usage

Hi all.

Hope this is not too convoluted but bare with me for a bit.

We currently run a Samsung tablet with no SIM card when we travel. Have EO Traveller on it using the tablet inbuilt GPS to log our travels.

We add points of interest and waypoints as we go to our personal places and wait till we get home to hook up to our wifi and upload all the info to the EO server.

We have traveller also on our apple iphone, enabling this to record our position. The phone sends our position in bundles of data to the EO server when we have Telstra coverage. Wait till we get home to save the file and name it.

Have had a few issues using the phone for EO Traveller losing positions from time to time resulting in a straight line display rather than following the road. Has happened when we were using the phone with Google maps at the same time and on one occasion when we received a phone call.

We were given a Telstra prepaid 4GX Wi-Fi Plus device that has a pre-paid sim card when we moved into our new house. Telstra had issues hooking up our landline so this was to use till the landline issue was sorted.

Still have the device but haven't used it for some years. Thinking about maybe getting a prepaid SIM for it and using it in the car hooking up our tablet to it by WiFi. Tablet and Telstra device would then send our tracking data to the EO server without needing to use the phone.

All the above has eventually gets me to my question. If the SIM in my Telstra device is being used solely for EO Tracking on my tablet, how much data would I need to purchase for say a 28 day period. Our last trip covered around 5500K in 21 days. I realise each trip would be different but is it possible to get a Kb/Km value I could use to sort out data useage.
Thanks,