Here are the rules
http://www.sccu.ndo.co.uk/increment.htm
from which
and35 moves in 1 hour 40 minutes followed by all moves in 20 minutes; with an increment of 10 seconds for every move played from the start
I don't think you should do an add-on in increment mode without counting clock presses.The move counter is not in operation, as errors by players can lead to confusion.
The 20-minute add-on will occur automatically, on both clocks, as soon as the initial time period of 1hr 40min has expired on one of the clocks. A static black flag will appear on that clock when this has happened. Players should then check that the required 35 moves have been made.
Lets take an example. The time available for 30 moves is 100 minutes plus 300 seconds, so 105 minutes. The above rule seems to imply that the extra 20 minutes would be added on once a player got to 5 minutes remaining ( since the initial 100 minutes has expired ). This is totally useless since there's still 5 moves to go.
Perhaps what they really mean is that the extra 20 minutes will be added on once a clock hits zero. This is really quite dangerous to players since they could easily believe themselves to be seriously short of time with 20 minutes still in the bank, or could run themselves out of time in the belief that they still had another 20 minutes to add. (I've seen this happen in non-increment mode).