Platform arrival

If I have tickets for a reserved seat on a Nozomi train, how early do I need to be on the platform before the train departs?

You can be as near as 30 seconds before the train runs! I have even on-the-dot experiences with luggage to bring in many of my shinkansen rides and I got no problem!

Usually, the shinkansen stations have leeway for waiting. Sample of which is in Shin-Osaka since it is the start/stop for Sanyo/Tokaido/Kyushu lines where shinkansen stays for more or less than 5 minutes before its schedule. For sure this happens in Tokyo as well.

If the Hyperdia says you are allowed for this ample time to transfer from one platform to shinkansen platform, probably you can make it.

If you have reserved seats, then there is no need to arrive at platform too early. There are few bench seats on the platforms usually, where you can sit and wait, but most people wait inside where it's air conditioned and seats are more comfortable. Shinkansens do leave on time, rarely late and never early.

For trains at the departing station (like Tokyo or Shin-Osaka) the doors open around three minutes before departure. You cannot board before then so for a reserved seat, any earlier is waiting time.

Thank you to everyone who responded. You have answered my question perfectly.