how do i calculate share of voice properly?

For organic, the version most tools use: take a set list of keywords, estimate the clicks you'd get at your position on each one (search volume x expected CTR for that position), add it up, then divide by the total volume of the whole set. Do the same for competitors and compare. The bit that trips people up is the keyword list. Change the list and the number changes, so lock it and only compare month to month on the same set. For paid search just use impression share in Google Ads, it's already there.