Psychrometrics
The chart is the tool
Psychrometrics relates dry-bulb temperature (DB), humidity ratio (W), relative humidity (RH), dew point (DP), wet-bulb (WB), enthalpy (h), and specific volume (v). Any two independent properties fix the state; the rest are read from the chart. Know that WB equals DP at saturation, that RH is a ratio of vapor pressures rather than a mass ratio, and that lines of constant WB are nearly lines of constant enthalpy.
Coil and airstream math
At sea level with air at 0.075 lb/ft³ the three standard formulas are: total heat = 4.5 × cfm × Δh; sensible heat = 1.08 × cfm × ΔT; latent heat = 0.68 × cfm × ΔW in grains/lb. All three scale with density, so correct them at altitude or for hot air. When two airstreams mix, the mixed state lies on the straight line between them, positioned by mass flow (approximately by cfm at similar density), not by geometric distance.
Traps
Confusing RH with humidity ratio; assuming a coil leaves air at its apparatus dew point (bypass factor says otherwise); treating evaporative cooling as constant-enthalpy without checking saturation; and using 1.08 far from standard conditions.
Worked example
1,000 cfm at 80°F DB, 67°F WB is cooled to 58°F DB, 57°F WB. Enthalpies are roughly 31.5 and 24.5 Btu/lb, so total load = 4.5 × 1,000 × 7 = 31,500 Btu/h, about 2.6 tons. Sensible = 1.08 × 1,000 × 22 = 23,760 Btu/h; the remainder is latent.