Carbide bore bars deflect a very small amount. Even a 1.2mm carbide bar, that is 6mm long, 5d, only deflects less than 0.01mm cutting material at 52Rc. A 5 or 6mm bar at 5d only deflects microns.
My normal way of setting the bore bar is to take a cut across the end face on a piece of brass or Al. I have the tool deliberately under the centre line. When faced, it leaves a little spigot of material. Measure the diameter of the spigot. With a DTI on the tool , I move it up 1/2 of the diameter plus 0.01mm. With the very small bars like the Ø1.2 bars, I blue the end of the brass or ali bar, and use the phone as a magnifier to see the diameter of the spot and if the tool went above , below or through the middle. 0.05mm is very visible with the magnifier app.
With my lathes, they all have the same Dickson Myford QCTP. I set the other lathes with a spacer, so that any tool that is correctly set is always on centreline or where I want it to be. Keeping the surfaces very clean is the important aspect of the Dickson holders and main block.
Preparation is the key to any job, and sorting the tooling ahead of time, or job planning is essential. I have several tool holders with the most common tools that cover the majority of work I am going to do. The micro bars, are in their dedicated holders with the adapters. I really like the Iscar Picco series of small boring bars. Readily available, and they have common shank sizes for a series tools that are held by a M4 grub screw or M5 grubscrew. The smaller than 4mm bars are on 4mm shank tools. So a 1.2mm bar can be changed to a 3.5mm bar, and they are on the same centreline still. The Iscar bars are about 4x or so of the import TiN coated carbide bars, but are a lot sharper and I get a lot longer cutting time from them.