I just wanted to stress to the OP that when people here say built-in they actually mean through-the-lens which is only one class of built-in meters. I shoot sometimes an olympus 35SP, great camera with good meter, and I have to compensate for the filter since the meter is not behind the filter. As for calculating the number of stops from filter factor, if you have a calculator just take the log2 (the logarithm to base 2) of the filter factor. If your calculator lacks the log2 and has only log10 (or just log) then use the following formula instead: No. of stops = log (filter factor) / log (2)

ex: filter factor = 8, then No. of stops = log2(8) = 3 = log (8) / log (2)