It is not really that simple, but normally, ease the mainsheet.

Use the upper batten/telltales for references for the combo of the mainsheet/vang/traveller.

Top batten not parallel, adjust the mainsheet, vang, and traveller until the tell tales fly and the top batten is close.

The farther out the mainsail is (off the centreline) the more important the vang becomes and the less important the traveller is.

 

As you have written the question, answer is false.
Assuming your sails where already trimmed for the beat....
First ease out the headsail for a beam reach and adjust lead...
Then lower the traveler, to keep the same mainsail shape.

if this is not enough then work the mainsheet and if present, vang, to get the shape correct. Upper batten could care less what position the traveler is in. leach twist is always controlled by sheet and vang.

no vang, no control of leach beyond end of traveler.

 

thanks for your input.