On our crew, I am the linesman. Like previously suggested, I look at our line judge and just "mimic" him. If he comes in killing the clock, then I know that we have a first down. If it's close, he will not kill the clock, but he will let the referee know that it's close. White hat then decides whether it's a first down (killing the clock), it's close and requires a measurement (again, killing the clock), or we are short (clock keeps running).

It's easier if you are on a field that has the yard lines marked every yard vs every 5 yards. Just about all the fields here are marked every yard. So, it's really easier for me to remember where the line to gain is.

Another suggestion, is to try to start every new series on a yard line. This makes it easier to judge where the line to gain is. Of course, again, this works great if the field is marked every yard vs every 5 yards.

Good luck.