the calculation is complicated, but i can give u a general one, which is GN = distance × f-number.
but for sony, its, given condition of F1, iso100, flashzoom105, and its 58meter.
the above is about how far it can reach*
but for "how far the effectiveness" is totally diff thing! there is calculation of light fallout...
i give u a PEEK if u understand...
» Click to show Spoiler - click again to hide... «
from the photo, lets study it.... (rayng will be happy on this)
u see i wrote F16 nearest to the flash, because that is the most intense light gathered, F16, iso100, so if u shoot bugs(incest, macro), and the flash is that near, if u set F11, ur photo will be overexpose by 1 stop, F16 is the accurate reading.
carry down, F8,..... F5.6.... F4... now, at F4 area, is where u set ur camera at F4, iso100, ur subject will be nicely lit!
now, new key point, u notice that, F4 area is larger then F8, that is light fallout, the distance it traveled, the light get less and less and less. right? light fall out also cause shadow fall out, the further u are, the slower the shadow fall out, if nearer u are to the source (F8 area) the faster the shadow fallout, hence harsher shadow (aka "burning tool")
now, here is something u must learn to ur advantage! knowning the light fallout lengthen after a distance... so if u tried to do a wedding portraiture with 2 person, or family group portraiture, by putting a distance of the flash give one advantage... let's see
if the group photo of 3 people, if i place them at point F8, F5.6 and F5.6, u will notice the first person is overly expose by 1 stop. but if i place the 3 at F4 placing, they are similiar within the F4 stop.
from the above statement, u learn this, when u shoot someone A with direct flash another person B is 2 step behind, u notice B is 1 or 2 stop under then the person A! if u take few more step back, and shoot, because of the light fallout lengthen, even the B is 2 feet behind the A guy, both will result in same exposure(more or less)
imagine the photo above is extended to 15 meter wide wall(sorry la, my house fence wall is short) and my flash power is set to full blast (forgot to mention the above blast is only 1/16)... then understand the below part...
that why, shoot group photo, or single portrait, within 3 meter distance, its easily have darken background,
but when shoot a stage event (10-15 meter away), the direct flash seem gentle on the peoples on stage... because the light is evenly spread over at that distance (10-15meter)...
sorry if i am bad in explaning... but try to catch it

for those who wonder, i never joined joemacnally or louis pang workshop (only attend sony basic workshop once, and now i am a trainer for them

), all this is some logical thing i notice during my photography journey, i got this understanding/idea of light at only 10 months into DSLR.... btw, the thing i mention above, i never seen any, ANY book has written them yet. and i believe a lot ppl will damn blur what the hell i talking about >.<
i get what u mean...hehe...now i feel my flash isnt that hard to use compared to it before repair...damn good...but i got no idea what creative thing i can do with my flash..i also want to try out creative flash with only a single F42..
my attempt to be so-called creative..