First time v-neck, when to start decreasing and how many to decrease?

How-to Questions
1

Hiya,
So, it is my first time knitting a big project and I’ve gone and attempted it without a pattern (potentially a bad idea) but thought it was worth a shot if I keep it simple enough.
I’m attempting a diamond brocade knitted v-neck vest. I’ve gotten almost to the v-neck portion and am unsure when to start decreasing and how many to decrease.
As far as the arms are concerned I was just going to begin decreasing a few gradually until it looks right as I haven’t a clue what to do there either, unfortunately. Perhaps I have bitten off more than I can chew but hopefully it’s a good learning opportunity, haha. Any help greatly appreciated!!

20201230_114200
20201230_1142003456×4608 7.79 MB

Details
Gauge- 28x35 (over 4 inches- first time I’ve ever checked my gauge so I could very well be writing this wrong)
Needles- 4.5mm
Vest- full length (top of shoulder to bottom hem)= 50cm
Bottom of hem to bottom of vneck= 32cm (Hopefully)
Working over 65 stitches.
Back of neck measurement= 15/16cm
This leaves me with 18cm to work the neck over.

Diamond brocade details
Cast On: Multiples of 8, plus 1

  1. Row 1 (Right Side): * K4, P1, K3 *, K1
  2. Row 2 (Wrong Side): P1, * P2, K1, P1, K1, P3 *
  3. Row 3: * K2, P1, K3, P1, K1 *, K1
  4. Row 4: P1, * K1, P5, K1, P1 *
  5. Row 5: * P1, K7 *, P1
  6. Row 6: P1, * K1, P5, K1, P1 *
  7. Row 7: * K2, P1, K3, P1, K1 *, K1
  8. Row 8: P1, * P2, K1, P1, K1, P3 *

Thank you!!

2

Welcome to KnittingHelp!
That looks so pretty. Lovely pattern stitch.
You’re going through all the questions that a designer has to ask. These are often answered by trial and error.
You might look at similar patterns on Ravelry to get an idea of the shaping details. There’s also a book by Ann Budd that might help.
https://www.amazon.com/Knitters-Handy-Book-Sweater-Patterns/dp/1931499438
It’s a very well thought out book with templates for various kinds of sweater designs.