There seem to be several questions mingled in with those 2 questions.
1) Amps can go either way
2) Breakers trip when heat exceeds certain level. Heat is caused by amps.
3) You want to add a subpanel in garage.
I do not know the code in your area for installing a subpanel.
Your plan will work by connecting 6 gauge wire to main breaker, and new 60 amp breaker will protect wires between main box and subpanel.
Remember, wires going into main breaker cannot be turned off without pulling meter.
Some areas require license to pull meter.http://waterheatertimer.org/How-to-replace-circuit-breaker.html
4) To work around license and meter-pull, simply replace a 240V breaker in main panel with your new 60 amp breaker.
Then move breakers around to match new set-up.
New subpanel can accommodate two new 240V breakers and one new 120 Breakerhttp://waterheatertimer.org/How-to-install-a-subpanel.html
5) 6 gauge wire is correct for 60 amp breaker.
6) I'm not sure what you mean by the box rails?
7) Do you need more amperage on main service?
This means adding a larger service panel with 150 Watt or 200 Watt main breaker.
Some areas require service upgrade when remodeling or adding circuits.
8) How to figure total amp draw at your house.
Add up total watts being used.
For example you have 1/2 Hp motor
754 watts per Hp
1/2 Hp = 377Watts
Volts x Amps = Watts
Amps = Watts divided by Volts.
377 Watts divided by 240Volts = 1.57 amps (plus a bit more amperage when motor starts)
40 watt light bulb divided by 120Volts = .33 amps
Double oven has label located inside door that shows upper and lower wattage.
Dishwasher, dryer, big screen TV, satellite receiver all have labels that show wattage.
Water heater has label that shows wattage of each element.http://waterheatertimer.org/Figure-Volts-Amps-Watts-for-water-heater.htmlhttp://waterheatertimer.org/See-inside-main-breaker-box.html
Add a comment for more help