If you used the Slot tool to create the obround, you can use the centre point of the underlying sketch to position the mating hole. no extra sketch lines would be necessary.
If you didn't create the slot with the slot tool....
Why even convert entities? Just draw a construction line between the two radius centers in your hole wizard sketch and place your point at the midpoint of the construction line.
Sounds like you are doing way to much work.
I didn't make the part with the slot. .... but the suggestion about not bothering with the convert entities was really helpful. I don't know how I got into the habit of using convert entities but you are right that I don't need to do that. Just draw one construction line from the centerponts of the obrounds and use the hole wizard to pick the midpoint of the construction line. Nice.