To dose excel just follow the instructions on the bottle.
There are many ways to double(over) dose excel to kill algae. You can use 1x the initial(waterchange) dose and double the daily amount according to the label's directions. You can double both of the doses listed on the bottle's label. I usually just dose 1x the initial dose every time I put it in my tank and dont use the smaller daily dose. I read that excel only lasts like 24 hours in the tank before it is decomposed. Just dont overdo it--like 3 or 4 X the dose listed on the bottle.

Good luck!

-Mike B-