The "odd" percentage, as you cal it, is necessary due to the fact that; since I am a very busy person, I do not have the liberty of taking all the time to prime and restart my fiter, nor do I want to increase my chances of burning out the filter by my own fault.
the tank's lenght is 20"
the width is 10.25"
and the height of all the water taken out amounts to 3"
why three inches? simple, it is the lowest i can go w/ out having to reprime the filter.
That would be fine, IMO. I do the same thing with my 5 gallon tank. Approx. 25% PWC w/ gravel vac on Sundays and Wednesdays. I do my 35 gallon tank on Wednesdays only.
Being that your tank isn't particularly overstocked and that it is planted, one PWC of 27% once a week should be fine. The fish you have listed are not huge waste producers. Two PWC of that amount each week would be more than enough if you prefer to do it that way.