w/o calculator :
Therefore to work out prayer times round the year inside your head, you need to work out basically two things.
1. Equation of time.
2. A sinusoidal function where the amplitude is 23.5 and period 365 days. And origin is at March 21st.
The rest are only two additions and a single multiplication to find the time.
Next we shall try to figure out how much accuracy we get on these two values using simpler solutions.
Obviously the simpler the solution, the less accurate the number.