for the simplicity of not bothering with any logical or engineering that seems to be this thread.....
for the very general application of a relay or solenoid, one will get a back EMF spike of not more than 5 times the forward voltage. sorry, but you cannot discuss a coil and EMF without including reverse voltage. it is the only thing the diode is there for.
as for current, the back EMF spike will also not exceed 5 times the current from a relay or solenoid coil.
the reason is that the design of the relay or solenoid coil is such that the force needed are not that great.
so, in very simple applications, select a general purpose diode of not less than 5 times the forward voltage and 5 times the current.
there are lots of sites that you can use for calculations. however, you will need to know the data of the coil.
unfortunately, for many hobby parts, that information is not available.