Pre layering and sectioning question

My plastikhaar has arrived!! Yippie! It's really pretty and matches great with my hair. It's Dunklebraun 50 mm.

I plan on doing the layering technique where I pretip in different lengths before I install it. Since this is my very first go at extensions, I have no idea how many tips will go across each row. So I don't know how many to make before I start cutting this weft.

Can someone give me a ball park? I'm guessing like 20 from the top of ear to ear. And gradually less toward the nape of course. Is that even close to average or am I way off?

I'm installing with Shrinkies and I have fine/thin hair. The finished product will have lots of layers and be shoulder length tops if that helps.