This is an interesting question, because the first thought is to explain it to you a terms of programming methodology. But your question is about the language used in the statement:
The point here is you want to be as specific as it makes sense to be every chance you get.
Let me give some context of my own, first:
Cascading Style Sheets allow you to define styles in a hierarchical manner, relying on the higher levels to define a set of styles that apply to a broad range of rendered elements. For instance, you may specify overall font styles, including font type, font color, and font height, as examples.
At the lower levels in the style sheet definitions, you may modify the styles for more specific elements. So you may want to change the font weight for an element, to emphasize a word, for example.
What your sentence is telling you is that you do not need to respecify all of the other details. Only specify the style element that is needed. This is what would make sense. This is all that is necessary. To do otherwise would (or may) create additional work, it would be unnecessary (obviously), and may even lead to other unexpected problems. To specify elements that have been adequately defined at the higher level would not make sense. There is no reason to specify style elements that are already specified the way you want them to be. (But you should respecify those that need to be changed.)
So to make sense in this case means to be reasonable or logical.
The point being made is that you should only be as specific as it reasonable (or logical) to be. Do this every chance you get.
You've kind of answered your own question. The second version of your statement is a perfect usage of hence.
Hence doesn't have to mean, in the future from now, but in the future of the events you've been speaking of so far.
Best Answer
I believe "as" can describe a sense of simultaneous occurrence whereas "when" gives a sense of sequential occurrence.
When I reached the house I saw some guy walk out the back door. 1. You reach the house, action completed. 2. AFTER you reached the house, you saw some guy.
As I reached the house I saw some guy walk out the back door. 1. As you are completing the action of reaching the house you see some guy.
It can also be used to create distance in time (when) vs something in passing (as).
When I was younger, I would unload my friend's truck on the farm since I was stronger. (in the past)
I saw my friend unloading his truck on the farm, but as I was younger, I told him I'd do it since I was stronger. (in the present)
But just then, as he reached into his pocket, I saw the bale straps give way. (simultaneous events)
Example of "when" and "as" in the same scene: He raised the gun to her head to draw further attention to his apparent intent. With her body trembling, all she could think of was at this distance any attempt to escape would be futile. She knew that when he pulls that trigger it would already be too late. He tightens his grip to get ready for the kick but as he pulls the trigger the girl suddenly drops dead. A faint whirr and a glottal stop - the bullet piercing her spinal column - all from a trajectory that can be traced back to a rear window. "There he is!". He rushes to the window "Hey you there! Stop!". The assassin quickly disappears into the night, aided by an already fastened rope the dark figure dances over the adjoining apartment wall. Time to give chase. It's a long, long way down Spunkmeyer. Fortunately when things get this desperate, jumping out of an open window on the fifth floor doesn't seem so crazy. He looks back at his target. Her corpse lay limp on the floor. A growing red stain radiating from the exit wound.