Analogue gauges arent accurate enough to measure full vacuum, they will change with weather conditions and elevation so they aren't too useful.

A better indication is the tone of the pump. You will notice even a tiny leak in just the change in the noise the pump makes when you clamp and unclamp the line. If you can clamp the line for half an hour and then unclamp it and not notice a difference in the pumps operation, then you can be pretty confident you've got no leaks. 

Hi, struggling to get perfect seal. So tried clamping off the line straight into the catch pot but shouldn't this be ready - 30?

Thanks