I was trying to figure out how to calculate an ascent rate of a helium balloon, and I thought to calculate it's buoyant force $F_b$, subtract it from the gravitational force $mg$, and find some kind of effective $g$; call it $g_e$. Obviously, this value is negative.
The 'iffy' point was where I took that value, made it positive, and put it in the terminal velocity formula (modelling the balloon as a sphere), instead of $g$. Despite getting fairly decent results, I wasn't sure how physically correct using that formula was. According to this website, the drag formula for fluids work for slow moving objects (slower than I'm assuming ~5m, hence why I didn't use it).
Side note: According to a research paper I found, up to ~35km, balloons ascend at more or less a constant rate. Not strictly relevant, but it gives context as to why I want to find its terminal velocity.