John, do you have a boring head?
I made a solid carbide centre out of a broken centre drill, and fit that in the boring head in lieu of a tool-bit.
Mount the head in the tailstock horizontally, and you can dial in the offset directly.
I guess you could also mount a suitable sized ball bearing in a bit of bar, and use that instead.

Bill

Have you seen the Hemingway kit 

Hi John,

I assume you are looking for something like this:

Link

or this.

Thor

I think someone in the past posted calculations (in relation to general vertical and horizontal tailstock alignment) that showed the horizontalness of the boring head is not super critical, so just ordinary care is needed to put it into the tailstock for an acceptable result.

Something to assist this might be a block that goes between the top slide and the flat on the head.

If length were an issue, the Morse arbor on the boring head could be removed and an adaptor that clamps it to the OD of the tailstock barrel be made. That might incorporate a key, to interface with the keyway on the barrel.

I made (in 1986) this adjustable  offset revolving centre when I needed to turn a number of tapered shafts.  Since then it only gets occasional use.  Recently used by a friend to machine a Brown and Sharpe taper.

For the occasional job a boring head is entirely adequate, I would not bother doing as I did, unless turning tapers became a regular job, as it was for me back then.

If anyone want the boring head dead level then all you need do is stuff a bit of 1/2″ or 12mm round bar in the cross hole and stick a digital angle block or your engineers level against that.

Editor Neil has an article in the current ME&W about fluting simple columns. If you have any questions just ask, as I have done some larger ones.