Where are the rules for shooting Flyers from the ground?

Let's say that my player's Explorator shoots incoming Vendetta - where are the modifiers to hit such fast, flying target? We have bonus from Size and Range (probably Extremely Long), but is there anything else? I can't find it anywhere, even in other rulebooks.

-Distance +size modifier -relative speed (-10 per 40km/h of difference?)

Well, Orca has a Cruising Speed of 1400, so it's simply impossible to hit it on full speed unless you have Hydra. However, it's Tactical Speed is only 30m/Turn (30m/4 second, 7.5/s, it's 27 km/h), so there would be no penalty for shooting it aside from Range. Is that correct?

To be honest, it seems a little weird, that shooting fast-moving troop transport, created to be hard to hit, isn't even that hard.

On the other hand - I can't find this in rules anywhere. How long it takes for Tau Battlesuits to drop from 600m to the ground? 1 turn? That would make their speed 150m/s... 540 km/h, they would be ripped to shreds. So, anyone found the exact rules? Or maybe you have any idea how long it should take?
Also, what would the penalty for shooting deep-striking Crisis/Stealth Suit?

By the way... the same question applies to every other deep-striking unit. How fast are Stormtroopers/Elysians descending? How fast are SM/CSM descending on their Jump Packs? Aside from Drop Pods in DW, we have no speed listed for deep striking from flyers...

Sorry for shameless bumping, but... anyone? Anything?

Sorry, but this is exactly the type of thing I avoid getting specific about in my games. I throw out a modifier and if the players disagree they are welcome to give their reasoning. If their reasoning is sound, then we compromise.