you mean the error was you were compensating for 3 stops? im glad you got it figured out ^_^

for the record when determining equivalent exposures anytime you change iso and the number doubles you have increased the sensetivity by one whole stop, any time the number is cut in half that is one stop down. each time you change iso by a stop up you must decrease your aperture or shutter by a full stop to compensate (or you can adjust both at partial stops as long as it adds up to a full stop per ISO stop increase) when you stop down on your ISO you must open aperture or increase shutter a full stop to the ratio of how many stops you have stopped down on your iso. this is why when making equivalent exposures theres really no guess work involved.

now as i tried to state before there can be occasionally reason why an equivalent exposure wouldnt work i'll now get a tad more in depth on that matter. i dont think so much on a point and shoot but for slr use, large zoom lenses have more elements inside them and when zoomed out all the way more light falls off as it travels to the film plane/sensor so compensation must occur and an equivalent exposure would cause some underexposure to a degree. i believe there are other instances when things can occur to cause the equivalent to be "wrong" but i cant think of them off the top of my head... something else to think about is that everyone's meters arent exactly the same i recomend you do a series of tests to decide if your meter meters right on, or if it has you over or underexpose slightly.

another point i would like to bring up is that i feel point and shoot cameras with manual features are questionable. i was recently playing with a canon powershot a95 which is my parents p and s. when using a flash on an slr (and should be true of any camera) aperture controls the amount of flash that is captured in the exposure (because of the fact that the flash burts so quickly shutter speed does not come into play). now with that in mind shutter speed allows control of how much ambient light effects the exposure so flash and ambient light can be balanced to taste based on these factors. if i want an image where the flash effects it very little i close aperture quite a bit so it would be fairly underexposed if using a fast shutter speed, essentially that means the flash will have little effect on the lighting on the final image. now the issue i have with the p&s was that the flash exposure wasnt changing when i was supposedly changing aperture *shrug* (sure has me confused) but when i changed the ISO on the p&s it acted very much like if i was simply changing an aperture. this has me wondering if the point and shoot cameras are really changing aperture size when it says iso, and is doing something more like what a sensor does when changing iso when the camera says im adjusting aperture. i dont think i will ever totally figure this out (nor do i care to make an effort *and maybe this is a canon thing*) and it doesnt necessarily effect anyone unless they are trying to use a flash but ultimately my point is if you can ever afford it i would suggest upgrading your backup camera when possible. sorry this turned into such a long response. kudos to anyone who actually read it all