First thing i will see the default spacing. If each letter are too close, I will add the letter spacing by 0.3px, and if it's still too close i will add to 0.5px. For example Poppins, the default letter spacing (0px) for me is too close and 0.5px is the best.

For uppercase i always love to give more space on each letter, since the scanning for each letter is harder than lowercase, it's because uppercase have same height for each letter. You can see the comparison below how more space increase readibility on uppercase word.

Line height is the space between each row on a paragaraph. For the line height I have 2 methods to find good line height on a paragraph.

First method is to add 10px from font size. So if i use 16px for font size, the line height would be 26px. If i use 14px for font size, the line height would be 24px.

Second method is using 1.5x from font size. So if i use 16px for font size, the line height would be 24px. If i use 14px for font size, the line height would be 21px.

Which method that i will use is depend on the result. Usually i will try both methods and compare the result then decide which one is looking good and impact the readibility.