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Lynn RNKnitWit for shared her version: Alpaca Boucle Brown/Gray Shawl She worked one inc beg every third row to 50 st, then dec in same manner.

I did that and at 37 st, had 40” or so length and 22” or so depth. Plenty long and half of the carried along sparkly yarn was gone.
Now looking at it, Ms. Lynn’s shawl looks like her increases / decreases occurred more frequently than one every third. Also, it is important to do all the increases / decreases along one edge only. If you add them on both edges, you get a long diamond shape instead of a triangle.

Mine is a very gradual elongated diamond, but it’s okay. The fabric is very flexible. I thought I would be working a number of rows even in the center, but that would have made it too long.

Two sk Ultimo used for main scarf, st st.
Third sk worked around outer edge crochet.
Omitted fringe.
The scarf is plenty long for most people. Looping the ends into a simple knot helps control the length and looks fabulous.

Recommendation
If you elect to omit fringe. Work two or three st ea side in garter st so the side edges don’t curl in

Keep track of each inc row at the edge with a locking marker. The row needing the next inc is easily apparent then.

About the increases:
Make a sample casting on 3 stitches.
Work up 4 rows.
Commence increases as desired and see if you’re getting the triangle shape you like. (Increasing one st at beg and end of same row will make stair step edges.)

Increasing one on alternate edges ( every other, every third, or every fourth…) makes for more gradual edges and a symmetric scarf. The more frequently the increases are made, the more quickly the scarf will become wider.

Increasing on one edge only results in an asymmetric triangle. Both are great. It’s just a matter of what you like.

Ultimo winds up easily using a ball winder.