Calculation issue in virtual feed

Hi created a new virtual feed, this feed should calculate the subtraction between other 2 feed.

I added:

Source Feed MyFeed1    (feed last value 27.69)

- feed MyFeed2               (feed last value 21.13)

My expectation is that result on my virtual feed should be 6,53, but the value of virtual feed is -21.13

It seems like that value of Source Feed is not used in calculation,

Whats I miss?

 

Thanks