Float Level

I have a Culligan Estate -M bought in Feb-2000.
I am not sure how large my brine tank is (160, 250, 375)
it is 42" high, 59" Cir, 18" Dia
I think we have soft water because it lathers up well with soap there is just my wife and I (73 & 71), It will
recharge about every five days. I have it set to after 500 gallons
it will use 12 lbs of salt to recharge for 10 minutes.
I am not sure to where to set the float, I have looked at the manual
but I don't know the size of the brine tank for which chart to use.
Thank you very much for your help.