Finding the best temp for ufh

[Declan52]

So am of work for a week and bored already so had a thought about trying to find the best temperature for the water in my ufh.Currently it's set at 36. The plumber set it at this and never changed it in 4 years. 

My set up is a pellet boiler which heats a 350l thermal store to 68 degrees and from this I draw all my heat for both my ufh and 3 rads on the first floor and all my dhw. Each room on the ground floor has a stat which calls for heat when needed.

I am trying to figure out is it better/ more economical to have water at its present temp being pumped round for the 90 mins it generally takes to start to feel heat in your feet or would it be better to reduce this temp to somewhere between 25-30. Once the slab is heated it takes  a few hrs to cool down. 

So would it be more efficient to run the heating for 3-4 hrs at 25 degrees or keep it as it sits. Going to 25 it will obviously take longer to empty the thermal store so the stove won't fire up as much. But the pumps etc will be running for much longer. 

During the winter I would have the heating come on once in the morning and if it gets really cold then once in the evening.