Here is a method of constructing a variable flush conversion for about $10. Not only does it provide substantial savings on your water bill over a period of a year, it is a very environmentally friendly project.
Step 1: Tool, parts and planning
1) 1/8 drill bit
3) hack saw
4) hobby knife
5) locking pliers
6) multi tool/pliers and assorted small tools
9) tape measure
optional: scissors if you your multi tool doesn't have them
1) 2" ABS/PVC plastic pipe
2) acrylic sheet 1/8 or more
3) coat hanger
4) plastic container (see notes)
5) toilet flapper
6) waterproof epoxy
optional: super glue and latex glue
planing: Start by measuring the inside of the tank to see where and what size jar to use and where to place it. If there is a large float that takes too much space then you should move it or replace it. The new system will work by having the plastic jar holding some water in the tank for a small flush and for a big flush the flapper in the jar is opened to allow all water to drain.
notes: For the plastic container I used a P.E.T (Polyethylene Terephthalate) plastic jar, I found out that it resists glue, it still bonds to the plastic and preforms the job, however, it's not very strong, so AVOID anything with ethylene in its name it's better to use containers made of acrylic aluminum, PVC, lexan or ABS, you can I dentine the type of plastic. The easiest way to tell is to see if it snaps when bent, then it is good. If you cant find a jar then make one with a large peace of pvc and a pipe cap.