Calculating folding equity on a semi bluff

[Malachii]

How do you calculate how often an opponent would have to fold when you're semi bluffing for the play to be +EV?

Example 1:

There's $75 in the pot after your opponent bets $25. You have an OESD and have roughly %17 equity in the pot with one card to come. You opt to make a potsized raise, investing a $25 call + a $75, hoping to represent a set. How often does your opponent have to fold here for this play to be +EV?

Example 2:

Same hand as example 1, except you intend to bet another $150 on the river if your opponent calls your 4th street raise if you hit or miss with your straight. How often does this play have to work in order to be +EV? How do I calculate this?

If someone could please explain to me the formula for calculating this, I would really appreciate it. Thank you!