RMS calculation through Rectangular Window with 25% time gap

From any signal (e.g. sine), I need to calculate the RMS value, with a 1 millisecond integration time, through a Rectangular Window, with a 25% time gap.
In other words, instead of calculating an RMS value by considering every 1 ms interval, I need to calculate the RMS value of a 1 ms window, then apply a 25% (of the window) time gap and then calculate the next RMS value by considering a 1 ms interval, and so on.

So, in a single 1 ms window, there is a 75% superposition.

I appreciate any help.

Thank you in advance.