According to RuiRaptor, (https://www.youtube.com/watch?v=RhlTdTLfaQ0) for a Tevo Tornado, with MKS Gen L, with TMC2208's on X-Y-Z and an LV8729 on the extruder, the Vref should be as follows:
X - 1.19
Y - 1.26
Z - 1.19
Extruder:
LV8729 with R100 resistor - 0.50
LV8729 with R200 resistor - 0.99