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a1

Clearance

b1

b2

A1

B1

B2

Space allocation height

    A1 = Height of item_2  + Clearance
     ∴  a1 = Clearance
     
    B1 + B2 + Clearance = A1
    b1 = B1 - Height of item_4
    b2 = B2 - Height of item_5
    b1 + b2 = B1 + B2 - Height of item_4 - Height of item_5
            =  A1 - Clearance - Height of item_4 - Height of item_5
    ∴  b1 = b2 = (A1 - Clearance - Height of item_4 - Height of item_5) / 2