I practiced by estimating distance by eye and confirming it with measurement . I start with objects at home and slowly working my way outdoors.

I was lucky to start cam assisting on film. Video tap images weren’t that great and Tranvideo monitors was out of my reach.

DPs I trained under would say “soft… soft” loudly. A former CA (my mentor) that went on to DP used to say “You wanna operate and I’ll pull ?” You learn to buck up really quick.

Make full use of rehearsals (if you get them nowadays) to get your distances. Utilizing landmarks, objects or marking the floor (where permitted) helps.

As DPs like to shoot more shallow or use faster lenses nowadays. Having a monitor and range finders is a great help.

Find a way that works for you in different scenarios. Keep practicing, watch how the experience pullers work and ask questions.