I would add about 25% as a buffer even though it seems like a lot better to have enough room then not enough.

You can also always try the following site as well to calculate - Veeam backup capacity calculator

do you have any references for adding 25% as a buffer ?

because I suggest only 5% for buffer.

this Veeam backup capacity calculator has added buffer ?

I don’t have any references as that number is subjective to each person.

Yes, that other link has the buffer which is called “Workspace” at the bottom.  That is the amount of buffer you need.

is buffer and workspace same ?

because if I use  https://rps.dewin.me/rpc, it also has workspace, and how many percent of  Veeam backup capacity calculator use as buffer ?

Workspace = Buffer

I don’t know what they use - you need to input numbers and then see what the Workspace value is then you can calculate the % from that.

it helps me a lot. thank you.

You are welcome.  Glad to assist.