First of all, thank you gillian for your response. Your support is always appreciated. Now to answer your queries :-

But . . . just to clarify, If I post a comment on someone's blog and nobody upvotes it then there will be no payout on it. Is that correct?

Yes, there will be no payout. Actually when steemit was created, there used to be a reward for author who wrote parent blog, which was somewhat 25% of the earnings of commentators. Now that reward quota has been removed (i dont exactly know in which hardfork they did so). So now your comments are 100% yours. If you earn upvotes on your comments, you will be paid, if your comments do not receive any upvote, payout will be zero.

Also, I wasn't aware of the 75/25 split.

It isn’t exactly 75/25 split. It depends. Generally it is 75/25. For instance if you have written a post and you get an upvote seconds after posting it, you will get 100% of the upvote. If you get an upvote after 15 minutes, the split is somewhat 85-15. After 30 minutes it is 75-25. With each passing minute leading upto first 30 minutes, the percentage will change. However, beyond that, it is 75-25%.

And one more point of clarification . . . if I reply to a comment on my own blog then that reply becomes a post in it's own right. Is that correct?

Yes that is correct. Each comment you post, no matter where (that includes replying to your own comment too), will be considered as a post itself.

I hope that i have answered all of your queries, if there is still left any, please by all means, do not hesitate to send it my way ^_^