Ic-racer, no, no need of any calculator.
You can set the f-stop interval anytime in 1/100, 1/10, 1/4, 1/3, 1/2 or full stop. Once done (I use 1/10 by default ) you may adjust your time whenever you want, directly by rotating the encoder, no need to confirm anything, just set the f-stop/time and start the timer.
Once you have done a test strip, no need to set your time again to make your exposure, just select the strip of your choice and expose, nothing more.
Hope to be clear enough.....