I've interpreted HI! as H(I!) and have a completed grid, so feel confident in saying that's correct. A quick online search suggests factorial takes precedence over multiplication by convention.
For those who have been put off, take heart: it looks horrid at first, with its carte blanche and non-integer answers, but I'd say it's no harder than any other puzzle of the map-letters-to-numbers type. Maybe even easier than some other examples of the genre. That sounds like bragging on the part of this solver, but I think it's a fair assessment - the puzzle just has an intimidating exterior. (I won't say *why* I think it's approachable as that would give too much away.)