The rule of thumb is to start off at 70% of your RM for a given exercise. If it is 2.5 kgs, then start off with 1.5 kgs and do:
1.5, 1.5, 2.0, 2.0, 2.5, 2.5
If you do not have those DB's, then do 2.5 all the way through... or even find something else you can lift that is lighter.
You don't...