New 36 inch exterior prehung door in garage.

[DesertRider]

I’m installing a new 36x80 prehung door in my sons garage, unfortunately the floor is NOT level... over the 36 inch length one end is 3/8 lower. The door has an aluminum threshold that I want to support it’s full length... any suggestions for shimming?
The aluminum is rather lightweight and bends really easily... would a shim the end and grout the full length work or is the a better way?

 

[Snoonyb]

Shims are "generally" about 1/4" at the thick end, so set in a bed of aggregate caulk, should suffice.