So for starters, there are two V60 "setups".
-One where the cone is elevated on a brew bar of some sort- with this the scale is reading only your final product, not applied water to filter.
-The second your cup, cone, filter and coffee are all tared out ontop of your scale. With this, your scale is reading the total applied water, both extracted (in cup) and extracting (in cone).

How exactly do you achieve a specific brew ratio for the second option? Say you have a brew bar, it's easy. Dose 24g and cut your pourover once you see you have 350g in cup (or whatever your desired ratio is.)

Just stop pouring when you reach your ratio and wait for it to drain. It's that simple.

Oh that was what I was thinking, thanks.

Coffee grounds absorb and retain up to 2x its weight when brewed. So, by using the 2nd method, your yield will be: total mL poured minus mL retained. Does this make sense?

As for brewing ratios, this refers to the total amount of water brewed rather than the final yield. So for example, when someone brews at a 1:18 ratio the math would look something like this:

Grounds weight: 20g

Water poured: 360 mL

Yield: Somewhere between 300-330mL depending on whether the brewer let all the liquid drain completely or stopped short a little before as the final drops are understood by some to be more bitter.

The common brew ratios (16:1 - 18:1ish) have the 2x bean weight absorption baked in, so there's probably no need to adjust your ratio, unless you find one that tastes better.