I did a little job for Dave Bluechip a couple of weeks ago, attempting to straighten out an ER collet fitting. I set the topslide to the normal rough 8 degrees and after grinding, it was found to be spot on, no further tweaking of the topslide required.
So really, all you do is set the topslide with the rough angle markings, and when checked out with engineers (prussian) blue after the first few shallow cuts, it is only just a tiny tweak either way to get it spot on.
I honestly think you are maybe clouding the issue by attempting to get it spot on first time and turning it into a major machining exercise rather than just a normal turning job.
Once set up for cutting at 8 degs, the whole process only takes 10 to 15 minutes, and that is with fine adjusting as well, maybe a little longer because you are not used to doing such things.
Just relax, don't panic, think about what you are doing and take your time. You will kick yourself when you find out just how easy it is to do when you do everything in logical steps.
So basically, measure the small end of the collet, and with a drill just a tiny bit under or even spot on that size, drill down the part as deep as you want to go.
Find the largest boring bar that you have that will go down the drilled hole, and set it up and take a few very light skims down the hole, to make sure it is cutting OK.
Swing your topslide to 8 degs.
Wind your topslide fully back, and bring the tool tip to within about 5mm of entering the hole using the saddle and crosslide. Lock up the saddle.
Using the topslide start to take the cuts, using the crosslide to put the cut on.
Once you are about 2/3rds to depth with the cut, blue up and check your angle. Slacken off the topslide clamp screws so that they still grip a little, then tap the topslide very gently in the correct angular direction, using say the plastic handle of a screwdriver, then tighten up the nuts. Usually, if you see it move, you have gone too far, it really is only a tiny amount each time.
Take a skimming cut until all the blue has gone from the tapered hole, recheck for angle. Adjust if necessary.
Once you reach a full covering of blue, then you can gently open out until the collet sits perfectly at the correct depth in the hole.
Job done.
If you go a bit too deep, skim a couple of mm off the front of the job (you can use the set up boring bar), and then carry on taking material out of the hole, but more gently this time.
This is a longer posting than I really wanted to do, but by explaining it this way does show that there is no black magic or super critical measurements involved to turning an inside taper.
By all means go ahead and try to get things spot on first time, but it could cost you a lot more money in time, effort and tooling costs than the tool you are making is worth. The way I have explained is how a jobbing machinist gets it done in less time than it takes the average home machinist to even think about doing it.
All that is required is a tin or tube of Engineers Blue (NOT marking out blue) and a bit of logical thinking, plus of course, common sense.
ALWAYS TRY TO GO FOR THE EASY OPTION.
Bogs