It can be done in many ways
1) Using Look up Calculation
2) Using Separate calculation
3) Using Difference
Could you please attach a sample workbook so that it will be easy to understand your data and provide the solution.
Do you mean like this ?
I didn't bother with the accuracy of the calculation because I just made them up, but if you drop this onto your viz and then change the mark to shape, this will add a shape mark to your viz. You can then enhance this by adding a number or some text around it using the label mark as well as adjusting the shape to anything you need.
HTH 10.2 attached
Tableau Test_v10.2.twbx 50.7 KB
I'm really look for something like this:
For item A - would like a green arrow showing that the percent increased from 0.1% to 0.3%