You have a figure that is close but not exact, and the real amount could fall either side of it.
序 · The move
1Name the number and its measure word as a whole count, the way you would for an exact figure.Is this a near-figure you are rounding to, with the truth possibly above or below it?
2Park 左右 after the complete number-and-measure.Does 左右 sit at the tail, with nothing of the count wedged inside it?
3Read it back as a window straddling the figure, not a remainder only on top.Could the real amount be a touch under the number as well as over it?
例 · Examples
1他每天搭車take a bus / ride上班go to work的時間time, duration需要need, take (an amount)一個小時one hour — the round count左右or so, give or take — a window around the figure。
His daily commute by bus takes about an hour.
界 · Boundary
數+多+量
多 wedges inside the count and only adds on top — 一個多小時 is over an hour, never under. 左右 trails the whole count and opens both sides — 一個小時左右 can be a little short of an hour or a little past it. 多 leans one way; 左右 straddles.
幾 stands in place of a digit you cannot fix (幾個 = some, the slot empty). 左右 keeps the number stated and brackets it from outside (一百個左右 = around a hundred). 幾 replaces the count; 左右 wraps a stated one.
✗ 左右一個小時 → ✓ 一個小時左右 (左右 follows the figure, it does not lead it)
✗ 大概一個小時多 for 'about an hour' → ✓ 一個小時左右 (多 only goes over; for a two-sided 'about', use 左右)
English floats the approximation in front as a loose word ('about an hour', 'roughly a hundred'); Chinese hangs 左右 on the tail after the count is complete, so the learner reaches for a front adverb and forgets the bracket goes last.