Pretty elementary question here, but I'm new to astronomy and I'm having trouble estimating LST. Online sources give me the exact formula, but this doesn't really help me on the fly. My location is 97.7-ish degrees W of Greenwich, which I know means I can subtract roughly 5.5 hours from Sidereal time at Greenwich. I know that at the vernal equinox in march is when the ecliptic cross the equator, solar noon at Greenwich (lat. 0) = RA of 0, and the autumnal equinox in September is at an RA of 12h, so the change between sidereal time and solar time is about 4 minutes per day or about a degree. I can kind of picture how this is happening qualitatively, but I'm kind of lost on roughly quantifying it for a particular day/time/location. My professor will say something like "Ok, it's September 17th, and it's 8:15pm here, which means it's 1:15am in Greenwich, so roughly what is our LST?" Obviously he's just looking for a rough estimate within a few minutes or so of the actual LST, so how do I go about doing this in my head? Thanks!