lease line bandwidth requirement based on wireshark

According to your screenshot you have an average bit rate of 103kbps, so 512 would be enough.

But that is only valid if you can be sure that

1. you captured the traffic long enough to have all up and down peaks covered (e.g. if you capture in the night when nobody is using the lines the values aren't that relevant)
2. Averages tend to hide peaks, so it would be better to look at the I/O graph in the statistics menu to check what the peaks are, and base your requirements on those instead of averages.