I'd be fine with requiring a 60% vote of the house, or votes representing 60% of the population. 40% of the senate represents less than 12% of the US population.
The Senate exists for one reason only: The slaveholding governors of Carolinas, Georgia, et. al would never have ratified the constitution if it was thought that slavery had a snowball's chance in hell of being abolished by the federal government.
Interestingly the Australian Senate was modelled on the US Senate, in as much as it was intended to give the smaller states a brake on federal power (and similarly was really a necessary carrot in order to get the smaller states on board with federation).
It's now a directly elected proportional-representation body, so 60% of Senators in that body would tend to represent a similar proportion of voters.
The Senate exists for one reason only: The slaveholding governors of Carolinas, Georgia, et. al would never have ratified the constitution if it was thought that slavery had a snowball's chance in hell of being abolished by the federal government.