With BnW the temp of the chemistry is not as important as having a consistent temp. Room temp is fine. I would always mix the chems up the day before and let them stabilize at room temp and use them there, whatever temp there was. You don't want to start off warm and them let it cool down as you print and vice/versa. You only want to fight one thing at a time. Keep everything stable and adjust only one thing. Use the same chems, the same paper, the same f stop, etc. just vary the time. After you get good consistent results and understand which variable does what, then you can branch out.
When I did test strips I would leave the time at 5 sec, (or 2), then cover all but one inch. Expose, then move the cover over 1 inch, expose and move again. That way you never touch anything but the start button and the cover. That way the last strip gets only 5 sec and each step has exactly a 5 sec difference.