How to adjust a fonts for good 'centering'

[Vanals]

Hi Capsize team!

I am currently working on the NewsKit design system and we are trying to adopt the Capsize library for cropping our fonts.
We had some issues when using a Poppins font file, a google font we converted from ttf to woff/2. When using it the text ended up not being centered in the container even if we were using the metrics provided from the CapSize website.

Once used the woff/2 directly provided by Google, instead, we noticed the fonts were finally well centered.

We assume the conversion might have altered the metrics and we needed to adjust the metric manually. Instead the woff/2 files we then downloaded from google came with a nice adjustment already.

• Our converted Poppins font file loaded in your web site:

I have actually found some specification about how the ttf has been converted using FontSquirrel

{"mode":"expert","formats":["woff","woff2"],"tt_instructor":"default","fix_gasp":"xy","fix_vertical_metrics":"Y","metrics_ascent":"","metrics_descent":"","metrics_linegap":"","add_spaces":"Y","add_hyphens":"Y","fallback_custom":"100","fallback":"arial","options_subset":"basic","subset_custom":"","subset_custom_range":"","subset_ot_features_list":"","css_stylesheet":"stylesheet.css","filename_suffix":"-webfont","emsquare":"2048","spacing_adjustment":"0"}

Maybe you can spot some settings might have lead to a wrong adjustment :).

• Same Popoppins font(same weight etc) directly downloaded from google. It doesn't look right on the website but once used the font is the same and was better centered in our containers.

I can see there is a bit of spacing at the top of the H< I guess we should adjust that little bit :)?

Adjusting the font metrics is something also our users might need to do.

So, is there a general guideline for how a font should be adjusted? In order to be uniform, I guess, and centered? without any other particular need.

For the moment I am assuming the His what must be within the Cap height lines as in your default example.
Some of us were wondering, what about the g? which looks a bit over. But I guess as far as the H is like in your default example should be ok?

And finally, the Em Square we believe should provide uniform spacing (green lines in the screenshots) on top and bottom between the Cap Height lined

Looking forward for some feedback and if you can more info for a guideline! Thank you.