To keep things simple, just maintain at least the cross-sectional flow area that the louvered vent provides. Let's say it's 12x24". That gives you a square area of 288in^2. We'll disregard the louvres for this example.
Example 1: A box in front of the vent
Plan (top-down) view:
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If you were to place a box-shaped object in front of the vent, you'd take the length of it, plus twice the height (it has two sides around which air can flow), and divide that sum into 288. For example, say you place a box that's 24" high by 36" wide, you'd calculate like so:
288 / (24 + 24 + 36) = 288 / 84 = 4.43
This tells us that, in order to maintain roughly the same flow as you have now through that vent, you'd need to keep your box 4.43 inches away from the wall.
Example 2: Full-width cabinet front of the vent
Plan (top-down) view:
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In the case where your obstruction extends tight to the sidewalls, you'd simply calculate the length and width of the opening at the top of the obstruction, between it and the wall. Say your hallway is 32" wide, you'd calculate like so:
288 / 32 = 9
This tells us that you should leave 9" of space behind your cabinet (or whatever).
Caveats
Now, this is all a bit oversimplified as the furnace ideally draws from down low, near the floor, to scavenge the coolest air available. If you move your flow up hear the ceiling, you reduce some efficiency of the system, or at least some of the mixing effect of the system.
We're also disregarding the additional complexity of the airflow path, which creates some resistance. By adding a bit to our calculated distances we can reduce flow loss from bends in the flow to negligible amounts.
If your box-shaped thing has legs, leaving open flow below, be sure to include this area in any calculations.
