I am still stuck. Below shows my working out – and confusion - in detail. Spoon, if this is derailing then please ask a mod to split this into a new topic in the BC.

Quick recap:

The way I was taught to calculate post odds:
to call / pot x 100. So to call $10 to win a pot of $25 is 10/25 x 100 = 40%.
10/25 is 10 in 25 (1 in 2.5). And "IN" is always 1 more than "TO".

In regards to Spoon’s dilemma in the OP, I calculated that if we have 42% equity then calling a bet of 40% is just about EV+.

The next post covers this situation as well as calling a $10 pot sized bet and calling a $10 bet into a $40 pot – examples discussed above.