The pattern lists three fabrics I do not have and I have two metres of something I love — how do you judge a substitution?

There is a method and it is simpler than the fabric names suggest. Ignore the fibre and match the behaviour.

The envelope lists poplin, chambray and linen — three different fibres that behave the same way: mid-weight, crisp, holds a fold, does not cling. That shared behaviour is what the pattern was drafted around, not the fibre content. So the question is not "is viscose twill allowed", it is "does this behave like those".

Three tests, all of which take a minute:

The drape test. Hang a length over your hand or the edge of a table. Crisp fabrics stand out and make a wide, shallow curve. Drapey ones fall almost straight down in narrow folds. Compare against a shirt you own made of poplin. Yours will fall much further, and that tells you how far off you are.

The scrunch test. Grab a handful and let go. Crisp fabrics hold creases and spring back into a shape; drapey ones fall out flat.

The pin-to-yourself test. Pin a piece to your shoulder and look in a mirror. This is the one that actually answers the question, because it shows you what the garment will do rather than what the fabric does.

Your viscose twill will make a real garment. It will be a softer, closer, more fluid dress than the one on the envelope — a different design, not a failure.

Adding the part that turns "different" into "wrong", because there is one specific way this goes badly and it is predictable.

Ease. A pattern is drafted with a certain amount of room built in for the fabric it expects. A crisp fabric holds that room away from the body as shape. A drapey fabric does not hold anything — it collapses onto you, so the same amount of ease reads as excess fabric clinging in places you did not intend.

So a shirt dress in a fluid fabric often wants less ease than the pattern has, particularly through the bust and waist, or it looks like a garment two sizes too big rather than a soft one. That is the adjustment you were asking whether existed.

The other places to watch on this specific substitution:

Gathers and pleats. Drafted for a crisp fabric they will be crisper and stand out; in your twill they will fall flat and soft. Usually fine, sometimes the whole point of the design and now missing.

Collars, cuffs and plackets. These rely on body. In a fluid fabric they go limp. This is fixable — a firmer interfacing than the pattern calls for restores the structure exactly where the design needs it, and it is the single most useful adjustment for your dress.

Buttons and buttonholes. A soft fabric gapes more between them. Interfacing the button band well matters more than it would in poplin.