While I'm sure that there is a better spread for venusaur than the one I have down currently, bearing in mind that it was just plucked out of the air, I don't think looking for very specific benchmarks is the way to find it either. It's essentially impossible to know what the true "best spread" is because this game is far too complicated. By all means, we should consider whether we want a physical or special bias, and we should possibly consider jump points, but not benchmarks such as that one. The game is complicated enough that the distribution of amounts of damage venusaur will be taking can be seen as continuous, especially bearing in mind the damage variation on each move.
I disagree. I think that specific benchmarks such as this are exactly what we need to keep in mind because they translate into real-life scenarios. Let's just say, for argument's sake, we decide that a physically defensive bias is the best for Mega Venusaur. You still want to have enough special bulk to handle some of the special threats that teams often rely on Mega Venusaur to handle. I picked Greninja here because it's a pretty common offensive threat, and many teams often rely on Mega Venusaur as their first line of defense against it. That said, being able to always escape a 2HKO from non-Extrasensory Greninja after Stealth Rock (a very common field condition) is a valuable trait. Sure, Mega Venusaur won't always be at full health, but it's not unusual at all for it to take some damage over the course of the match, heal back up to full health via Synthesis/Leech Seed/Giga Drain, switch out, and then have to switch in to take on Greninja, possibly with Stealth Rock down. Not only that, but this specific benchmark doesn't take much investment to achieve. The second spread in my last post requires only 44 Def EVs to be moved to SpD in order to survive the aforementioned 2HKO, which gives Mega Venusaur the ability to safely take on a common special threat even with Stealth Rock down while keeping its physical bulk mostly intact.
These sorts of specific damage benchmarks are very common in our analyses for a reason. If you look through many of the analyses in this sub-forum with very specific EV spreads (not just your average 252/252/4 spread), you'll probably see a comment like, "The given defensive EVs allow Pokemon A to escape a XHKO from Pokemon B," or, "The given offensive EVs allow Pokemon A to always XHKO Pokemon B." We pick these specific benchmarks because they give the spread a purpose that plays out in actual matches. It certainly won't always play out perfectly (many of these spreads rely on the Pokemon being at full health or switching into Stealth Rock with no prior damage), but they still do happen, and this gives the Pokemon in question insurance against the threats it's designed to check. Now, I'm not saying that the spread I posted is the single best one. Maybe non-Extrasensory Greninja isn't an important enough target to sacrifice physical bulk for. Heck, maybe a physically defensive bias isn't the best. However, I honestly believe that specific defensive benchmarks should be used so that we can design a Mega Venusaur spread that's capable of tanking what it needs to from one side of the offensive spectrum while still focusing most of its EVs on the other side.
If we aren't going for a specific benchmark, then I think it'd be best to just focus entirely on either a physically or specially defensive spread. I've found that splitting EVs without a specific purpose sometimes makes it harder for Mega Venusaur to wall opponents because it's trying to handle too much at once. Essentially, when you try too hard to wall everything, you find it more difficult to wall anything.