First find out the shrink rate of your clay, your supplier will have marked it on the bag, or phone them and ask (my usual clay shrinks 12-15% as written on the bag)........
........OR do a shrink test.......roll out a strip of clay, mark out a 10cm (4") line with 1cm (3/8") markings on it, fire it to C10. Remeasure the whole line to find out how much it shrank, this will give you a percentage result.
Cut out a thin strip of paper, wrap it around your finger and put a mark at the point where the 2 parts come together, this gives you the circumference measurement of your finger. Lay the strip flat and measure the circumference as a straight line measurement.
Compare the paper line measurement to the fired clay line measurement and ADD THE EXTRA PERCENTAGE AMOUNT you found out from your clay bag, clay supplier or your shrink test firing. This is how I have done mine.