How to estimate rate of Trans Log usage
You didn't specify which SQL version you are on but you could use a Server Side trace or an Extended events session to capture the traffic that is sent to the SQL server (if you can live with the performance impact it might have) and replay those captured traces on a test environment to see what the daily growth would be for the day where you ran your traces.

See here how to replay a profiler trace or distributed replay for info on how to run an XEvents replay.

That would allow you to rerun the captured workload on a test system and see for real what is happening with your log file.