For the previous poster, to get at what Hermes said..........it has taken me about two months to finally get within a much nicer % of my road bike power on my TT bike. A few months of mostly TT riding with a few road rides mixed in. It will never be 1:1 IMO simply because a seated climbing position affords so much flexibility in how you can sit, squirm, move, change position, and breath that it's the way it is.
I remember before this, in TT position, I couldn't get within 100w of my road position 2min power. Maybe not a good example, but 100w is a boatload.
That took 3 months of TT riding and 2 months of a lot of TT specific workouts to bridge the gap. Even then, it's not 1:1.
Having been through it myself, instead of "easing into" the position as you do the work.........I'd go ahead and just do a mock TT outside or on Zwift to get a "TT bike" ftp. Then use that for your TT bike workouts in the position you intend to use.