As the amount of offset is determined by the overal length of the part, ctrs, ctr hole depth, etc you will have to go back to school boy maths or CAD to work things out.

You should be able to get quite close by running a dti against your blank for a known distance, 2″ of travel should show a taper of 0.1004″ on dia, half that on actual reading. Adjust the offset to get that reading. Turn part and then blue and check in a socket, adjust a srequired.

Is it the type which goes in the tailstock?
I used a boring head for that job with a carbide centre I made, though the principal is the same; I now have a Myford taper turning attachment which makes life easier.
The problem is that the required offset will vary according to the length of the workpiece.
Essentially its basic trigonometry, but the workpiece length is critical to the taper attachment’s offset.
Rather than calculating I set the offset manually.

Set up a commercial blank between the centres thin end to the tailstock.
Dial in with a clock in the toolpost to run parallel with the taper.
Take a test cut and fine tune with engineer’s blue to get the exact offset.

Repeat for each blank, whilst you have the attachment set.

I didn’t have a blank to copy, hence the longer procedure below;
The reason I made double ended ones was so I has enough metal to attach a drive dog, and I wasn’t sure how long I would later need for the blank parallel section(s).

I set up a length of metal with a centre drilling sticking out of the 3 jaw chuck, and used that to trap a commercial MT2 centre between it and the taper offset gizmo in the tailstock.
Projection length is a bit over  half the length of the intended twin arbour; i.e. length of a centre plus twice the blank part.
I could then set the taper angle with a dial gauge in the toolpost after trapping the commercial centre, thin end at the tailstock.
Remove the metal bar from the chuck and replace with a short pointed bar to act as a centre.
It needed a bit of thought to set the correct projection exactly, but roughly as above.
Turn and fine tune a taper.
Reverse the workpiece and cut the other end.

Bill