You guys are way overspending and overcomplicating this ... how about buying one of
these for half the price of a cheapo vacuum gauge and set it to switch on an LED on the panel when it gets to 5" HG or 10".
If you really splurge and spend almost 2/3 of what Racor wants for a T mount gauge, you can have three of them and a green LED for when the filter is clean, a yellow for when it starts to get dirty, and a red for when it makes you nervous.