Depends on your putter. With my putter i find that anything below tourney greens i have to have multiple adjustments for the different scales. Everyone has different factors and how they calculate but the way i was told to do it was dividing the distance by 1.2 for VF, then adding a foot for every inch uphill and subtracting for downhill works. Well on Very fast greens that doesn't work with my putter. On my 10 foot scale i have to just hit it straight up. So if it is level and 8 feet i hit it 8ft sometimes a bit more. When i move to my 20 foot i have to do about the same up to about14 or 15 feet then i can take a foot or so off, then once i get to my 30 foot scale i actually divide by less then 1.2, more like 1.15 or 1.18. Find those all that works perfectly for me.
So for me if i had that putt i would have hit for 13 feet, cuz i wouldn't have subtracted anything for VF and i would have added a foot for the uphill. Hitting firmer is always better, IMO better to be 2-4 feet long than even 2 inches short. You can't every sink a putt if it lands short and pretty much anything inside 4 feet should be makeable. Plus hitting firmer can sometimes help ball keep line better, although you said there was no break.
Then there is variation which i am not even going to get into.
Live and learn i guess