"offset" option in the LEN(gth) menu. hold ALT while turning the encoder to change the offset of the starting stage. " I modulated "offset" last time i used Metropolix , pretty cool to have this as an modulation destination.
"Skip" function also is fun, skip the stages until the one you want to start from and then "You can quickly swap which steps are skipped and which are un-skipped if you assign
“Skip Invert” as the destination of a CTRL knob, AUX Input (X, Y or Z) input or a MOD Lane." (from the Manual)