There's a guide here....
http://www.bpassoc.org.uk/BloodPressureandyou/ Thebasics
It can depend what is normal for you. The usually stated norm on 120 over 80 but 90/60 is the level below which is is said to be too low.
People can have more naturally high or low blood pressure, low blood pressure runs in the family and mine is often lower than yours but I do experience faintness when it is really low.
What your normal blood pressure is can also make a difference with the drops, if it's naturally low then a drop may not have such as drastic effect as someone with high blood pressure dropping quickly to low.