Problems w/ Nest

I’ve got a common problem but I can’t seem to find the simple answer. My heating system is simple. Forced water oil boiler with three zones. Two zones are controlled by the Nest E Thermostat. I’m having issues with those two zones. When the tstat is no longer calling for heat, the zone valve gets stuck. I’m thinking that because I have no common wire on the tstats there isn’t enough power to shut those valves completely. The other zone with a battery operated "dumb" Honeywell works flawlessly.

I have simple wiring – red and white, but two wires are not used. I have a Hydrostat 3250 controller on the boiler so there’s no common terminal on the burner itself.

What I think I need to do is splice an extra wire from each of the Nest tstats to the white wire coming off of the "C" terminal on the transformer. This would allow for constant 24v power to the Nests. Before I go tearing into this I want to make sure I’m heading in the right direction.