For every 100g of PMDD you will need to add 500ml of water.
I suggest you only mix up only enough for 2-4 weeks supply. Keep the PPMD out of the light in a dry place.
For each ten gallons, try around 1/12 ml of PMDD a day (roughly 2 drops). A 29 gallon tank would then get 1/4 ml a day (roughly 6 drops). If you don't do many partial water changes, dose less. If you have a "high tech", densely planted, CO2 injected, VHO lit tank, you might want to double the dose to 1/6 ml of PMDD per 10 gallons a day (roughly 4 drops).
Just make sure you keep an eye on the Nitrate levels when using PMDD. Personally i try and keep it around 5ppm Nitrate. If over dosing this can lead to algae growth and other implications