One factor that seems to never get enough (or any) attention is landing surface. Where are you flying? or more importantly, where are you LANDING?
It seems as though all the "factory recommendations", charts, & calculations telling you what chute size you need are assuming best-case scenarios - meaning sod farms, nice cushy grass, or soft freshly tilled soil. But if you fly where I do - dry lake beds, sun-baked playa, desert plains, and such - you will quickly find that the recommended sizes are simply nowhere near big enough. Early in my rocketry life I went with recommended sizes (based on real/actual descent mass) and all too frequently saw damage on landings. This was, mind you, on rockets that many here would consider overbuilt and much "stronger" than necessary (at least where they fly).
I now upsize considerably on almost all my chutes - sometimes as much as double the "recommended" size. This of course makes packing much more challenging, but I'd rather spend more time flying and less repairing.
There are a lot of other factors/considerations when I'm choosing a chute size - wind conditions, available space, fin vulnerability, how important flight performance is, how far I'm willing to walk to recover, how "pretty" or costly the rocket is, etc. But in the end, landing surface tends to trump everything else to some degree, and I rarely find myself going with the recommended size, opting most of the time for bigger (or at least higher Cd). SLOW descent rates are good!
I also ended up making my own chutes, using VERY thin "calendered" ripstop and a high-Cd design. This allows me to shove "more chute" into a given available space, which helps a lot.
Another way of honing in on "just the right size" is to over-size, then fly successive smaller sizes until you find what you can get away with. This of course assumes you have a sizable fleet of chutes to choose from, which I've been fortunate enough to acquire over time.
s6
