one question left and just cant find the answer
how to determine the accuracy of an estimate of the size of a dna fragment using a standard curve on semilog graph paper
You should know the accuracy of the measurement before you plot the results on graph paper, using some sort of error analysis. Often "error bars" are used on a graph to indicate each data point's accuracy. See
http://peltiertech.com/Excel/ChartsHowTo/ErrorBars.html
for an example
How to perform an error analysis is a separate topic. Repeating measurements to determine reproducibility is one ingredient, and so is the precision of measurement devices.