Try creating a formula called somerthing like LastNameExtract.

 

In the formula use the following:

 

Left ({NameField},InStr ({NameField}, "," ) -1 )

 

The "InStr" function finds the "," in your {NameField} and returns the position of it in the string.

 

The "Left" function starts at the left of your string and stops at a specified position.

 

So, in the case of "Doe, Jane", the "InStr finds the "," in position 4 and the "Left" function starts in position 1 and stops in position 4,  - 1 to exclude the comma, which should be "Doe".

 

I hope that this helps.

 

Regards,