Whether you select "Strong Influence" or not, I have always used a weighted average for height, weight and age when calculating randomly, and used the min/max range when manually assigning (which I sometimes do to fit a specific character or NPC).
For your half-elf/half-halfing I would use:
* Age - adulthood = 50, lifespan = 340, increment 6.5
* Height - Male = 4' 3" base, Female = 4' base
* Weight - Male = 85 base modifier 1.5, Female = 70 base modifier 1.5
The reason why I said weighted average, is if you get a little crazy with it and go with a half-human, quarter elf, quarter dwarf.... (50% of human values + 25% of elven + 25% halfling)
* Age - adulthood = 34, lifespan = 220, increment 4.25
* Height - Male = 5' 1" base, Female = 4' 7" base
* Weight - Male = 132.5 base modifier 3.25, Female = 95 base modifier 2.75