Head tank switch

[DocsMapleSyrup]

I have a head tank that is 55 gallons. I would like to incorporate a float switch into to that when it gets low, it autofills and shuts off when full. How can I do that when my head tand is 30" tall? Also, I'm using a 12 v Rule bilge pump and I'd like the pump to turn on when there is about 20 gallons left in the in the head tank.

[psparr]

http://www.ebay.com/itm/TEMCo-Liquid-or-Water-Level-Float-Switch-Sensor-Sump-Fluid-Tank-NO-NC-Controller/370766378776?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%2 6asc%3D36188%26meid%3Ded58b2e201214c65a6d8baf085e9 1965%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D3213 72915240