https://drafts.csswg.org/css-inline/#initial-letter-wrapping says:

The rule for matching the character grid is 1ch × n + letter-spacing × (n - 1), where n is the size of the initial letter as specified by the initial-letter property.

This seems wrong on two accounts:

• This value is meant for CJK uses. The ic unit is the relevant one, not the ch unit. If we want to support aligning to a grid for latin monospaced fonts, we could make this depend on initial-letter-align, and use ic when it is set to ideographic and ch when it is set to alphabetic, but I am not sure it is worth doing, as Initial letters on monospaced latin text seems unusual.

• n shouldn't be the size of the initial letter as specified by the initial-letter property. That would result in a too small exclusion area when the line gap is large (or when the initial-letter text is long).

Here's an example of initial-letter: 2 with a large line gap in vertical text. Each [ ] represents a 1ic square.

[ ]   ____|____
[ ]   ' _____ '
[ ]         /
[ ]        /
[ ]    ---+---
[ ]       |
[ ]       |
[ ]       ’
[ ]   [ ]   [ ]
[ ]   [ ]   [ ]
[ ]   [ ]   [ ]
[ ]   [ ]   [ ]

Here, the exclusion area needs to be 8ic + 7 × letter-spacing, not 2ic + 1 × letter-spacing.

I suggest replacing that sentence with something like this:

The size of the exclusion area in the inline direction must be 1ic × n + letter-spacing × (n - 1) where n is the smallest integer for which the exclusion area is equal or larger than the advance measure of the initial letter.