the running line hitter-runner

[Spizzico]

Hi everybody,
I'm an italian umpire and if is possible i just wanna ask a suggest about a rule!Sorry about my english!
Which are the vision limits that an umpire-chief have to check to understand if the runner is or isn't in the running line.
The questions that i looking for an answer are:
if a player run with the left foot on the foul line,can be called interference or not?Do you look the runner's foot or all the body?

What I wanna understand,at the end, is when i have to call an interference!

PS I'm talking about the interference between the runner and the first base-man!

thanks!!!