Simply? Do it like this (arbitrary figured I am not 777 rated):
WEIGHT | SPEED
100t | 150kts
120t | 160kts
160 - 150 = 10.
10 / 200 = 0.05
0.05 x difference in weight above 100t in 100kg increments = Answer.
For example. Aircraft weighs 116.2t:
162 x 0.05 = 8.1
150 + 8.1 = 158.1 kts
V1 is therefore 158kts.
Horgy