The thing with Tracy's routines is that they are intended as guides only because the EASY routine is adapted to the individual LO.
For instance if you planned a 3hr 15 EASY and your LO woke from a nap early you might try to resettle in the cot for some time (we usually recommend no more than 45 mins) to establish habit and show LO this is nap time, but you might then feed a little earlier, say at 3hrs because if LO is not asleep she is likely to be using more calories (esp if crying) and be hungry earlier. There are examples in the book of Tracy feeding a LO a bit earlier than the intended routine during sleep training for this reason. As the day progresses you need to tweak timings a bit to fit in. You might have a slightly flexible BT too to ensure the last A time doesn't end up over-long.
As for fitting well into 12hrs - some babies just don't sleep a 12hr night no matter what you do. Mine only ever slept 10.5 to 11hrs over night and he was an independent sleeper from very early on (I started gentle sleep training from birth and started the EASY routine when he was 4-5wks old) so he had a longer day and shorter night but was well rested.
The last feed in the day is fine to be less than 3hr15 or even less than 3hrs. Some LOs do well with cluster feeding before BT (feeding a couple of times and at less than the regular 3hr E). I always fed mine at whatever time it was due after the last nap and again just before BT even if that turned out only to be an hour or so between feeds. I noticed he always took a big BT feed even if he had eaten not so long before.
When you transfer to the cot you will need to continue shush/pat until LO is in deep sleep. Tracy covers this in her books. Many parents experience LO going to sleep then stop shush/pat and leave them only to discover they wake 20 mins (or less) later. This is when they transition between light and deep sleep and is a time they can wake. If you pat all the way to the deep sleep phase you will likely have more success - it takes time - Tracy said EASY is not necessarily easy, put in the time and effort and it pays off though.
If you like to begin the wind down routine on the chair that's fine, it's the last part of the process which matters when you make changes. The putting into the cot and supporting Lo whilst they learn to sleep in the cot - then learn to sleep with less help from you.
Here's a bit of info:Shush-pat - How to
hope this helps - let us know if you have more questions