I wouldn't cut a test bar as the first thing - what I'd do would be to level the bed of the lathe first with something like a Starrett 98 (accurate to .005"/ft) then maybe tweak with a precision level using something like the Starrett 199 (accurate to .0005"/ft). After that center the tailstock (it's adjustable) to the center of the spindle using either a parallel bar or a 2 ring cut on a bar (a couple of feet of 2" stock between centers with the middle 22" relieved, cut ends rings with a light cut until a small cut with a sharp tool takes a light cut on the ends without changing tool position on carriage). The latter centers the center of the tailstock with the lathe's spindle axis.
Only after knowing how level the bed is can you start checking the accuracy of the carriage travel on the bed of the lathe, and making tweaks to adapt to errors. Know that you can't remove errors by twisting the bed or shifting the tailstock, all you can do is to minimize them in one area of the bed by making them greater in another. Further, you can't usually average out the error over the whole bed, to remove .006"/ft taper near the headstock you might have to introduce .010"/ft error further out from the headstock.
Usually after leveling the bed and getting the tailstock to center that you'll find that the majority of the remaining error is close to the headstock. I've seen lathes that had error distributed over the bed but that's a rare condition. Turning close to the chuck is the cause and if you're doing similar work you can't do a lot to avoid it. One lathe I rebuilt had .005" taper in a 6" bore from that exact sort of wear, and to avoid the majority of the wear I started working with more sitckout on the work and tool, moving the carriage travel to a part of the bed with less wear (until I rebuilt it to about .0002" over 20")
BTW: if the lathe bed was warped in its last installation you might need to wait for it to settle out in unwarping. If you have a foot up in the air give it a couple of days to fall by itself. That also means that you probably want to check for level for some time after leveling the lathe.