Stack measurement help
Stack and reach are a 'system' of alternative bike frame measurements always used together. They are supposed to be meaningful absolute values, unlike seat tube length (or 'effective' seat tune length, where top tubes are not horizontal). 

They are based on two datum points: the bottom bracket axle axis and the top of the head tube. Stack is the vertical distance between those two, reach is the horizontal distance.

If you have a meter long ruler (or a tape measure and something long and straight) and a level you can measure the stack and reach of your existing bike and make a comparison to the Slate geometry.