So, our Kill Team has been very tactically poor forever, but it has reached the point where my character has had enough. He is going to take defensive preparations into his own hands from this point onward, and once a sound defensive position is established, then and only then proceed with his technosorcery. This naturally leads to a few questions:

How long should it reasonably take to create cover (barricades etc), and what is a fair value for that cover? I know it depends on the environment. Still- assuming you're in a ship or building, and there is no tables or other cover around, so you're literally tearing bulkheads or walls out and repurposing them. Let's say too that you are in a rocky or earthen environment, and you need to dig out trenches or embankments. What are reasonable time tables for that?

Next, the Techmarine ability to improve cover. It's a full action, it increases cover by an amount equal to his int bonus. There does not appear to be a limit to the number of pieces of cover it can be applied to. Nor does there appear to be a limit to how long it lasts. The thing that is not clear, though, is if it can be used repeatedly. Not stacking on top of itself, of course- I'm not saying that a Techmarine can spend 4 actions to add 4x his Int bonus to a piece of cover. However, when the cover is hit, and degraded, can it be used subsequently to bump the value of the cover back up?

Finally, the evaluate skill. If used once, let's say someone gets 1 degree of success. Can they later use it again in the same piece of cover, and get a greater number of degrees of success? If so, how would that work in relation to cover having degraded in the meantime?

No you cannot improve cover more than once. You cannot have an infinite well of cover.

Tactics involve more than just hiding behind that slab of metal (I blame "tactical cover based shooters" for this). Utilizing cover is not a bad idea though, but you aren't going to be digging a trench or building a bunker in a few rounds. It takes a while to do these things. Even space marines will take about 10 minutes to dig out a trench (assuming he brought a shovel, otherwise digging with your hands takes much longer). Cutting, moving, then welding or nailing other pieces is time consuming as well.

Pre-fab stuff could cut the time down a bit, but you've got to carry that stuff, and it can be pretty heavy.

Jeah, we had a lack of tactics in our group, too. It got better when the KT got their asses kicked regularly, because the enemys started using more tactics - kind of led to a rethinking.