I'm enjoying the V60 recipe from the last James Hoffmann book (The best coffee at home). 60 gr per liter. I experienced a lot with other recipes and ratios during the past few years, but this one is super easy to reproduce and makes great coffee every time you are awake enough to follow a 3 step guide. 😪

Does it even matter to adjust the ratio precisely if I don’t have a precision grinder and am using a Krups blade grinder?

Yes. Your coffee uniformity and your ratio are different things. You should check this video from the one and only Mr Hoffmann.
In any case, if you are enjoying your pourovers, you may want to invest in a better but cheap hand grinder. Timemore makes great products at an affordable price.

I'm using 60g/L, the best ratio in my opinion. I grind it using Timemore C2 and brew it using Hario Switch.

15 grams of water to 1 gram of beans normally. I might adjust it a little depending on the bean.

You reversed your coffee and water in the 12.5:1 portion.

Measuring is important for repeatability but getting a good grinder should be your highest priority. A Timemore C2 will be a huge improvement over the blade grinder. You should be able to find one for around $60 most places.

1:12.5 seems really high, imo. I've always been told 1:16-1:18 and get good results. I agree with the other comment in that a new grinder is what you should be considering