In order to defeat a defending army worth ~150 IPC, you’d need to spend ~200 IPC. I generally find this to be the case is for larger battles, say minimum of at least 10 defending units. Of course it varies by players aggressiveness or risk factor or a couple of other things.
This is true b/c it is extremely easy to defend. As you both point, Infantry are unmatched it that department.
So attackers need to outspend defenders ~4:3 in order to take. You need more if it is a combined attack, for example a UK-US 1-2, or perhaps less depending on how much you want to take with, or if you have lots of air, tanks can really move 2 spaces, or something else.
I look at it as simply an easy guideline, like usind a LL method to see if I have “avg” shot at taking, not a hard hitting “must play by” rule. I think of the same thing with the 3:1:1 army. It’s a nice guideline but rarely do I ever actually have that in games.
As for my army for 150, I’d probably have something like: