Offsetting a Lathe Tailstock

The Machinist's Calculator won't give you the tailstock offset, but will help you to calculate it.

Start off with your tailstock dead centre.

Hold a test piece between centres and take a cut at each end, making them exactly the same diameter.

For example, you're cutting a taper of 0.600"/ft (or 1:20 for a metric taper):
Set a dial indicator to zero right at the headstock end of the piece.
Using the taper calculator, enter the size of your test cut into the "Large Dia." text box. Let's say 1.010"

Move the indicator down to the tailstock end test cut, noting the length of travel, let's say 5.00". Enter that into the "Length" textbox.

Click "Calculate" and get the small diameter of 0.76". The offset is half the difference between the two diameters at a 5" length. In this case, (1.01 - 0.76)/2 = .125

Use the indicator on the test cut to measure moving the tailstock the correct offset. The piece might have moved a bit at the headstock end as well, so check each end after setting.

Don't forget that if your tailstock is not dead centre, either horizontally or vertically at the start, or if there is any twist in your lathe bed, the accuracy of the taper will be compromised.