Typically it depends on how much calcium based material that I want to melt. In this case, I will kinda swag it, but probably close to 15:1 - maybe 3 gallons of water and 1/5 to 1/4 gallon of muratic. 20:1 will do for less dirty stuff. If it is REALLY nasty stuff like covered in coralline, then 10:1.
I think that I read somewhere that titanium (assuming that your exchanger is titanium), is very resistant to HCL under 27-28%, so a 15:1 of the 20 baum 31% stuff from Lowes or Home Depot is plenty safe.
I will also leave that solution around and clean a bunch of other pumps and stuff too... no reason to waste it.