This is all to do with use of reference lines ... a reference line simply allows you to adjust the 'calculation' for a further condition ... and when using reference lines, we ALWAYS GO TO THE REF LINE FIRST, then adjust to the condition ... so for this problem:

1. Enter on the base scale at 450nm ... up to the ref line ... make a pencil mark.

2. Follow 'adjustment curve' back down till you meet the condition '50kt tail', make a pencil mark.

3. Now vertically up to the next section .... 45000kg Landing weight ... make a pencil mark.

4. Now across to the right hand scale .... 2500kg of fuel required.

.... remember, always to the ref line first, then adjust to the condition.