For most exercises ive used a ballpark value for 1Rm as a start. For example Squats i know my approx 5RM. Then added a few more kg. There are websites to estimate your 1RM from your 5, 10 or 12RM if you want to use a formula. Having done that, noted how Ive gone then adjusted up or down on the next workout depending on how I performed with the 5 sets. Even where I havent done an exercise eg 1 arm half row, its still possible to have an educated guess and adjust as you go. First week was a bit all over the place on a few exercises, but by the second week was up and running fine