FarmI wrote:

Yes. Looking for ideas on refill times and how far above the top of the
toilet cistern I need to place the mains top up float valve.

??? You asked about filling the cistern from your tank.

Correct, but when the tank level gets too low, I'll need to add water to
the tank to keep the toilet flushing until the tank it is topped up with
the next rain.

Having the mains top up float as low as possible means that I use as
little as possible mains water.

Do not confuse it with my greywater discussions.

Sydney Water has a campaign for people to connect their rainwater tanks
to their toilets, but they want you to buy and install an electric water
pressure pump to do this. Under their plan, you still have a mains top
up float value for when the tank gets low.

While I am happy to consider this, I realise that the cost of
electricty, pump, etc is far more than I will ever save in reduced mains
water cost, so forget it.

So I am looking at gravity feed version. It just means there is less of
the tank available and the mains top off float valve is installed higher.

In practice, unless we had a run of dry weather, the tap for the mains
top off would only be turned on when needed.

What I am trying to avoid is finding that 1' above cistern head allows
for one flush a day by the time it reflls.

If someone comes back and says that you really need 3', then it is a no
goer as not worth the effort with the existing tanks, unless I somehow
hit upon a very cheap supply of tall, thin water tanks that will fit up
the side path. (which has lost is effectiveness as a second side path
when the new "better insulated" water heater had to go outside).