Here's the "no calculation required" method:
Give it a whole second. Worst case you make the whole film black and have to try shorter time.
Best case you will get about 10 steps clearly readable.
Then you will want to adjust the time (you know the drill - two steps is one stop) until your negative test strip just barely fades to clear for 400 film (fades to clear about 4 steps earlier for 100).