While I've not use it on pondweed, I've used Reward the past 2 years for an Elodea outbreak in a simlar application process with a 15 gallon gas sprayer dragging weighted application hoses (hydraulic hoses). Works great just have to get the throttle down near idle or it pumps it out too quickly. I've been using a mix of 4oz Reward, 2oz Curtine Plus, 2 oz surfactant per gallon of water. Maximum I would personally go is 6oz Reward per gallon of water. Should be no need to go any stronger, it has been very effective at the ratios listed in my experience with it.
You are correct that you can empty that 15 gallon tank faster than you think (depending on your setup). Fill your system with water first and try it on land, Mine will spray water 10' out to the side of each hose at just above idle. Anymore throttle than that and really shoots it out! Having an extra hand in the boat while applying is a big help.
I can get our 2-acre lake done in about 4 tanks depending on the weed density an how fast I'm going. If you have an algea problem along with the pondweed the Cutrine Plus does well for a relatively small cost while you are applying the Reward. Good luck and any particular questions on my setup feel free to ask. I used the following link for my guide when building my setup:

http://plants.ifas.ufl.edu/manage/research-and-outreach/how-to-build-weighted-trailing-hoses

Water Fowl