The point about the milk delivery was that it was a regular occurrence and the milk boy's presence at the doorstep at that particular hour could be counted upon.
It was entirely fortuitous that Alan Close arrived and saw Julia when he did, most likely around 6.38pm.
He stated that he was running up to half an hour late that night because his bike, which he normally used, was out of action, so he had to do his round on foot. He would normally have arrived much earlier.
But Wallace did not arrive home from work until 6.05pm. He had even stayed chatting with a client and had a cup of tea with her.
In other words, if Close was supposed to be Wallace's 'alibi', Wallace deliberately ran the risk that Close and the 'alibi' would have been and gone even before he arrived home !
Wallace would have then had time to kill five people and no alibi at all.
If Close had arrived earlier, around his usual time, Wallace would have had the following problems:
a) no alibi, because he arrived home after Close had gone, or
b) Wallace would have had to leave the house suspiciously early, immediately after killing Julia (to make it seem he didn't have the time to do it), and arrive at Menlove Gardens suspiciously early. Or, more likely, if Wallace was guilty he would have arranged the fictitious appointment for 7pm, not 7.30pm, to "close the time gap", based on Wallace's assumption that Close would arrive a lot earlier than he did, in fact, arrive.
Alternatively, if Close had arrived later still, Wallace would have had the following problems:
c) he would have had to wait for Close to arrive to have an alibi, then kill Julia, and then arrive suspiciously late for his appointment at Menlove Gardens, or
d) abandon his one-time plan to kill Julia altogether, as too risky for the above reason.
As a matter of fact, it was established that William Herbert Wallace stepped off the tram at the corner of Menlove Gardens West at 7.20pm, at exactly the time an innocent man would have, with 10 minutes in hand to find what he thought would be an address in an adjoining street...
What are the chances that Alan "Goldilocks" Close would arrive just at the right time, not too early, not too late, to allow a guilty Wallace to seem like an innocent man with such perfect timing? For that is what happened.
And who in their right mind would base anything on such a plan working out?