I AM TRYING TO SIZE A DRYWELL FOR A TEN YEAR STORM OF 4" WITH AN IMPERVIOUS SURFACE COVER OF 4000 SQ FT. IF I REMEMBER CORRECTLY TH TOTAL RAIN RUNOFF WILL BE APPROX. 10,000 GALLONS. WHAT SIZE DRYWELL? ALSO, IF THE INFILTRATION TRENCH IS 4'X2'X75' THAT GIVES ME AN INFILTRATION VOLUME OF 6000...