My lo is about 7 and 1/2 months so I can relate to you wanting to drop the df. Currently, we have not tried and I don't plan on trying for a little bit longer. I think in your case, the biggest sign that your lo is ready to drop the df is by how many ozs they are drinking for their df. If they are still taking a full bottle, they may not be ready. Most mamas I know that bottlefeed have waited until they see a decrease in the df. I know this is going to be our plan... and last night he only took 3.5oz! wooohoo
As you mentioned, you can always cut the df back an oz every few days and see if he still makes it through the night. I think that is the best way and you seem to want to try it that way which is great. Eventually, the solids will take over the ozs in the df bottle, so I don't think it's a matter of replacing during the day like when they were younger and you wanted them to get through the night. But I may be wrong on this.
I'm not sure if that's the answer you wanted to hear, but I think it's best to follow your los cues.
HTH
I'd be interested in hearing what other mamas have to say, as I have not gone through with the process yet.