If you're talking the true lateral electric field, that's just Vds/Leff.
But at the drain, the gate-drain field is also an element of hot
carrier generation / steering / degradation (canonical worst case
HCE experiment bias is Vds=Vddmax, Vgs=Vddmax/2). The field
that drives all of that is the vector sum, pulling hot carriers into
the spacer oxide and gate ox.
Now determining just what Leff is, can be tricky (not helped by
an industry behavior of calling technologies by non-physical "L"
for marketing purposes).