Paul, I wouldn't worry too much about how much weight you're squatting as long as you are feeling it and working hard. If you've just upped your weight then you should give your body some time to adjust to the new demands. however you're feeling comfortable at the end of a track, or just a little tired rather than having to push yourself, then yes, it's time to up the weight, as long as you can still keep good form while doing it
My only suggestion would be to balance out the weight between your chest and your back - think of it from a point of view of balancing the two halves of your body, anterior and posterior.
If you DO decide to add on, I only ever advise increments of 1 at a time so that people don't slap on too much and risk injury.