Unfortunately, there is no such way to pinpoint the exact traffic a server can handle. This as you mentioned is dependant on many factors.
I’ll try to mention each factor that plays a big role in a server/droplet handling traffic. Firstly, let’s take a look at the server-side.
Server side-wise, the factor can be determined by two things, how well is the server optimized and what services it’s using. Usually, you have a type of SQL(MySQL)and a WebService (Apache/Nginx) at the very least. In regards to MySQL and how much the server can handle it depends on the databases, if they are not optimized a lot less than usual. The same goes regarding Apache/Nginx.
Having said the above, there are ways to optimize them and increase their productivity like using server-side caching services like OPcache. Additionally, regarding web services, you can use php-fpm for instance which again increases its capacity.
All in all, there is no formula as far as I’m aware that will help you with estimating this unless you know all the variables. My suggestion is always the same. Start with the smallest droplet and when you see the need, upgrade it to a bigger one.