For a coolant change, you should really do a chemical flush.
The first job that any coolant does ( in a clean system), is to line all metallic surfaces with a coating.
This coating needs to be removed if changing brands of coolant.
If that Calibre coolant has only 540g/L of Gylcol, then it isn't real good.
Any decent brand will have 900g/L and above.
Read the lables.....it is easier to use a Concentrate.
Most concentrates are 500ml makes 10 litres, so use 2 bottles, and it wont hurt.
I use Repco Concentrate Inhibitor, and have no hassles at all.
I also use Loctite Radiator cleaner as the chemical flush, also available from Repco.

Yes flushing with alkaline salts is pretty important. Tap water doesn't do too much to break down sludge, oils & scale in the system. I've flushed with water then flushed with a radiator cleaner and got about the same amount of crap came back out after it flushed clean with tap water.

50/50 is a bit over the top unless you live in the snow or drive a heavy duty diesel (F9000). Ford (and most other manufacturers) recommend 33% IIRC. I used to run my e series with no glycol. Just corrosion inhibitor & never had a problem with it. Glycol actually reduces the medias ability to transfer heat & I'm also pretty sure cams banned it from use in track cars/days.

Prestone coolant is guaranteed to mix with any colour coolant from any manufacturer and I usually use it but can't seem to find it around town anymore

As a premix it is good. 540g/L is 50:50.

Yes, Glycol does impead heat transfer.....but that is at 68% Glycol.....found this out from a Wynns Industrial Chemist.

That's at absolute point.... it starts impeding heat bleed off well before that. For every few degrees extra boiling point that you gain for the amount of glycol you add, you will see leaky hoses before any benefit at all over factory specs in my opinion.

Unless something in the heating system isn't operating as intended that is.

Not that I'm a chemist. I just read & have a little practical experience, with fords in particular, as no doubt you also do.

The coolant I am using is over 900g of that stuff .
Should I mix 50/50?