How do i decide the value of \$R\$,\$L\$ and \$K\$ in the bandgap when i want a specific \$V_{ref}\$
Assuming I1 = I2, then a K of 8 (meaning the emitter area of D2 is 8X the area of D1) provides log2(8)=3 * 18 milliVolts = 54 milliVolts across R.

You can scale up L*R to get close to what you want; the D3 provides some needed temperature compensation.