For sure you're not that far off. I'd also guess this translates to 11,3 cm or so.
I think the challenge isn't that much about absolute numbers anyways but more about the personal goals from each of us 🙂
Don't restrict yourself because of the price tag, the heels you already have are absolutely fine.
I wouldn't buy the Hot Chicks for practicing either, it's more of an coincidence that I can use them now because I already have them.
When I bought them I didn't even really think about wearing them, I just bought them for the looks. And for that it was hell a lot of money!