Considering how powerful and thought out Affinity Designer's grid features are can anyone think of a reason you wouldn't want a gutter on the left side? Other than an odd outlier here or there I can't think of any, just curious why Affinity designed it this way. Anyway, thanks for getting the math together and creating such a nice tool! I went ahead and expanded upon it a little to also let you calculate spacing and grid size if you'd like to have a full gutter to both sides instead of half. It's the difference between this:
Here's my updated spreadsheet.
My math skills could be a lot better but I did test this with several different values and always got the expected result, if anyone finds a problem please let me know and I'll do some troubleshooting.
I'm not a huge spreadsheet guy myself so I started by converting the calculations into Calca, if anyone else uses Calca and is interested here's the snippet. Note that I included a couple of other calculations and notes relevant to Affinity Designer as well but nothing too interesting.
Edit: Fixed my snippet link.