Calculating Total Cost Feed

I am on an economy 7 tarif
Using scheduals I have created feeds for Day_kwh and Night_kwh
Then I created virtual feeds for day_cost and night_cost (Day_kwh * Day Rate…)

I would like to have a virtual feed that added the day_cost + night cost
but it seems that virtual feeds cannot be sourced from other virtual feeds ?

Is there a way to get around this ?

The input feeds are an emontx monitoring Grid and Solar inverter
So my input processing is
grid → toKWH → log grid_kwh
solar->to KWH ->log solar_kwh
solar+grid → log use
use->toKWH->log use_kwh