You can calculate the angle on the worksheet, and include that in the
chart label. For example, with the following data in cells A2:B4 --

Jan  57
Feb  36
Mar  91

Insert a column between A and B
In cell B2, enter the formula:
  =A2& " " &TEXT(C2/SUM($C$2:$C$4)*360,"0.0")

Copy the formula down to cell A4
Select cells B2:C6, and create a pie chart, showing Category Name and
Value as labels.