24 C, which is room temperature for us. We could also do the measurements at 37 C, which is often cited as ‘standard conditions’, but it requires using an incubator and adds steps to the protocol. Also, for the RNA cases that we’ve checked, we get similar chemical reactivities for stable RNAs at 24C and 37 C, i.e. a successful design should work across a broad range of temperatures.
Might I suggest one more temp of 15 c the average surface temperature of Earth. Its the temp my brewing yest like to be at to work well and many other organism without a thermal regulatory system.