Given an estimation procedure and real data, how would one compute the mean squared error? What value represents the "true" realized volatility in the case of calculating the Mean Squared Error in estimation? I'm specifically interested in intraday estimation error (one minute trade data for example)?

Would it be incorrect to take a sequential 252 day period of intraday values to calculate the "true" realized volatility and compute the error against an estimate over a much smaller subset of these 252 days?