Currently you have to set the "start up delay per user" to achieve a full ramp up time delay (e.g. 300ms/100users=30secs). It would be nice to just set an actual value in "seconds to full ramp up" where the portal can just automatically calculate the "start up delay per user). This is specially helpful when doing multiple tests with different number of concurrent users.
This was released in version 3.5 (2015-03-27)