Firstly, do you need to calculate the RMS voltage on a cycle-by cycle basis? If it is to be read by a human, outputting 50 values a second is overkill. Can you not sample one cycle and process it out of the interrupt? This would involve storing the samples in a buffer. Why are you using a filter to remove the DC offset? Is the DC offset fixed? If so, you could use the max and min samples and determine the offset from these values.

(long) is a 'cast'. If sample.filtered is already a long, then is it probably not necessary. However the constants like 255 should be written 255L just to make sure the compiler knows what we want. A good book on 'c' should explain the rules.