Mixing your incubation medium is a good skill to learn, however since this is your first clutch there will be plenty of other things for you to worry about and "get right". Since you will only be having the one clutch I would recommend going with HatchRite.
It will take virtually all of the guess work out of getting the medium right. Just remember to weigh your egg container with the HatchRite in it before you add the eggs and again afterwards. Subtract the difference. This is the weight of your eggs.
To keep track of how much (if any) water has been lost in the medium through the incubation period you can refer back to the weight of everything (medium, container, and eggs combined). Once a week when you check on the eggs and open the lid (briefly) for a fresh air exchange, you can have your scale nearby along with a small cup of water. If the weight is low you can add small amounts of water (pour it in a corner to prevent getting any on the eggs) until the scale reads the same as your original reading.