2:3 ratio (SH:H2O is a 40% mix (2 gallons of SH out of 5 gallons of mix is 40%)). 10% DS'er pulls approximately 10% of your mix into the hose and mixes with 90% water, which would mean that your final ratio is around 4%:96% (SH:H2O). Some guys calculate mixing SH with Water as diluting the SH (i.e. reducing it's active chemical content), which is not accurate. It reduces the ratios thus making the mix less effective.
With that being said, how well does it work for you? I can't imagine your mix having much if any effect on a 100% cotton color shirt, much less anything else.
As to your question regarding the amount of SH & H2O in a 35 gallon mix, it's easy to figure out using your own numbers. there are 7 (5 gallons) in a 35 gallons, so 2 of those gallons will be SH, so 2 * 7 = 14 gallons of SH subtracted from 35 gallons leaves 21 gallons of water (3 * 7). The ratio is the same as in your 5 gallon bucket. Now, here's where the difference comes in. When you use an electric pump, you'll be pulling 100% of the mix (no DS'er) thereby increasing your mix to surface ratio to a true 40% mix. This ratio is probably where you want to start when doing roofs, lower for house and concrete (obviously that depends on how bad its condition).