[edit] Ooops, forgot to include momentum on this. Try again:
Const POT_SCALE# = 1.0/100.0
pot_val# = (value read from pot)
; Convert pot reading to value in range -1(left) to 1(right).
pot_x# = 1.0 - ((pot_val#-400) * POT_SCALE)
move# = (move# * 0.99) + (speed# * pot_x#)
x# = x# + move#