Is the thiosulfate solution you used at 0.05M concentration? If so, then the theoretical volume of the Na2S2O4 solution required is correct. Otherwise, you may have to check again your calculations.
Possibly the undershooting of your first three trials is due to the incomplete reaction of KIO3, KI and H2SO4 so that less I2 was produced, which as a result required less Na2S2O4 to reach the end point.
Another possibility is that your sampling from the 45 cm^3 solution of KIO3/KI/H2SO4 was not properly done. Did you well mix the solution before you pipetted 10 cm^3 of solution from it? Apparently, the overshooting of your fourth trial is due to that most of I2 was left in the solution.
The concentration of sodium thiosulphate in solution can be determined using an iodine titration. An iodine solution added to a sodium thiosulphate solution of unknown concentration will create oxidized iodine and tetrathionate ions. Starch is added to the sodium thiosulphate solution. Iodine is then added slowly. The solution will turn a dark blue color. When the dark blue disappears, it indicates the reaction is complete and the concentration of soduium thiosulphate can be calculated from the amount of tetrathionate ions that are produced and the amount of iodine solution used.
I recommend you do it by iodometric titration. Since domestic bleach is an oxidizer, it can oxidize iodide ion into iodine quantitatively. Then amount of iodine generated can be found by titrating against thiosulphate. Since I have not any standard method in my hand, I can only list out outline of what you need.
Apart from basic apparatus such as burette, pipette, Flasks and balance, you have to get potassuim iodide, starch solution and sodium thiosulphate. Starch solution is just an indicator for titration so it is not a standard solution. Potassium iodide may not be a standard since amount of iodine generated totally depends on the strength of bleach. But sodium thiosulphate is to be a primary...