Here is a little calculator to calculate the exposure with different settings from “f”, “sec” and “ISO”. I use this for my longtime picture by night, then the camera ends by 30sec. During the night you will may have one minutes ore even longer. The calculator works with all DSLR camera ore cameras with BULB function.
The calculator works from 1/8000sec til 30720sec, from ISO 50 til ISO 204800 and from f1 til f64.
Tip: for longer exposing over 30sec you will need a remote controller and your camera have to bee in the BULB mode. Details will you find in your specific camera manual for your DSLR