I heartily disagree. Go for the lowest rpm you can that will keep you rolling. It's lower than you think.
My 90's Japanese 1.6L 4-cylinder with similar gear ratios:
Runs steady speed just fine down to 25 in 5th gear (1000 rpm).
P&G driving, where I need a little more power, I limit 5th to 30 mph and above.
In your situation above, 35 in 4th or 43 in 5th, my answer would be 35 in 5th.
