Change resolution keep the same sensitivity

I currently play in 1920x1080 and would like to change to 1280x720 however when i do my mouse sensitivity is faster (if i play in fullscreen) ofcourse due to there being less pixels stretched into fullscreen. My question is does anyone know the math to keep the same sensitivity when changing resolution? Thanks to anyone who can help My sensitivity : 50 DPI/CPI : 1200
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