For ascent/descent quick reference, check out to the tooltip that displays when you hold your mouse over “Highlight glyphs under the mouse”. Then check how it goes for the example font.
For 6x12 you could likely use default ascent/descent with pixel size of 64. Then the optimal size will usually be either 12 or 16, depending on how software counts font sizes.
And some libraries just like to aggressively apply anti-aliasing when rasterizing glyphs for rendering so you may not always get what you want with a TTF/OTF font.