Oh right how interesting - I'd always thought it was just closest yard counted. Those commentators are quick then at working out especially when they go from 45 yards 9 inches own half to 47 yard and 2 inches on other half! My brain couldn't work that quickly!

It's true they just use an estimation when discussing what position they are at on the field.

"... at the 32 yard line".

That's often an estimate no matter the situation, unless they are about to score and you may hear someone say "...at the 1 foot line", even though there is no 1 foot line.

The more important number is yards to go for a first down. This is also usually approximated until it is short yardage (less than 1 or 2 yards).

You may hear "3rd and inches from the 24", or "4th and a foot from...", or "a yard and a half needed to pickup the first...", etc.

You may also hear something similar to "3rd down and forever" or "3rd and California" when there is an unusually long way to go to pickup the first, such as 3rd and 25 or something.

That's great thanks
So....

When I gets tackled where does the marker go -

Where his knee his the ground

Or

Where the ball is when his knee his the ground? (ie do they try and extend the ball out front? )

Where the ball is when his knee hits the ground (or elbow, or ****, or head, etc.).