partial roof/gutter calculation

I know one can calculate the entire runoff from a roof, but I want to know how much per gutter/downspout, so my assumption is that I need to perform separate calcs for different parts of the roof to determine how much flows "this" way and how much flows "that" way... am I in the ballpark?

Yes, you would have to measure the rough area of the sections of roof feeding different gutters. I believe it would be a bit more accurate to measure how much ground that area covers. If you have a low roof pitch then it wouldn't be too far off measuring the roof itself, but a steep roof would have a much larger surface area as opposed to how much area of ground it covers. Then again if you have a lot of strong wind from one direction then a roof facing that way may catch more rain and a roof facing away from it may not get as much rain as it would be blown over (less surface area facing the direction the rain comes from). It is usually better to underestimate and have extra than to expect much more than is realistic.

When you determine how much flows "this" way and "that" way remember that most gutters are sloped. My house has a hip roof (gutters on all four sides) so I have to consider which gutters slope to a particular downspout before I can calculate the amount of rain that reaches the downspout.

Thanks much, folks! I did a quick hose test but am still not 100% if I have this correct. North green/black gutter, SE corners of upper story orange/black and lower story yellow/blue, and the west green/orange are approximate guesses, even after a hose test... I simply couldn't determine for absolutely certain if the corners are the actual boundaries or if the pitch wraps around. Even if I am off a bit, from the looks of it, most of the total water is flowing to the SW and N of the property (orange, blue, and green downspouts) while a smaller amount flows into the backyard, which is where I wanted to harvest the water for beds, trees, vegetables, etc.

The green, black, and yellow downspouts all connect with buried PVC (indicated by the red dotted line)... the green downspout's PVC pipe is pitched to carry water all the way around the NE corner of the house where it turns south and connects with the black downspout and yellow downspout, all of which terminate in a single pop-up in the middle of the back yard. The colored arrows pointing away from the house indicate where I've disconnected the downspouts from their buried PVC and use plain old diverters... I had two ice dams several years ago (black and blue downspouts) that cracked open the PVC above the freeze line, which made me not trust what the plumbers (!) installed underground. So, as far as a rainwater harvesting plan, there is some existing infrastructure that I'm hoping could be used once I make the necessary repairs.

Presuming my map is more accurate than not, I'm wondering what my options might be for:

1) the orange downspout that currently empties via a garish flexible plastic downspout directly onto my driveway. I considered re-connecting it to the buried PVC and then building a rain garden in the front, but there is a massive black locust in the middle of the front which has roots all over the yard, which my guess would make a rain garden a challenge to implement. Another option might be to use the water to keep the full sun plants that face south and don't benefit from the shade of the locust nice and hydrated.
2) the blue downspout (once repaired and re-connected) that pops up about one 1/4 of the length of the driveway "sun" garden (as I call it, as this is a full sun bed). Water certainly benefitted the area closest to the pop-up when it was in active use, but I'm not sure how to use all that water along the entire length of the bed.
3) the green/black/yellow downspouts, which, if fully repaired and pitched properly, could empty into a back yard rain garden or swale system

Well, that's a lot to digest! Thanks again all!